Laser Diode Drive Requirements

The following must be achieved to properly drive a laser diode and not ruin
it in short order:

Absolute current limiting. This includes immunity to power line transients
as well as those that may occur during power-on and power-off cycling. The
parameters of many electronic components like ICs are rarely specified
during periods of changing input power. Special laser diode drive chips
are available which meet these requirements but a common op-amp may not be
suitable without extreme care in circuit design - if at all.

Current regulation. Efficiency and optical power output of a laser diode
goes up with decreasing temperature. This means that without optical
feedback, a laser diode switched on and adjusted at room temperature will
have reduced output once it warms up. Conversely, if the current is set up
after the laser diode has warmed up, it will likely blow out the next time
it is switched on at room temperature if there is no optical feedback
based regulation.

Note that the damage from improper drive is not only due to thermal effects
(though overheating is also possible) but due to exceeding the maximum optical
power density (E/M field gradients?) at one of the end facets (mirrors) - and
thus the nearly instantaneous nature of the risk.

The optical output of a laser diode also declines as it heats up. This is
reversible as long as no actual thermal damage has taken place. However,
facet damage due to exceeding the optical output specifications is permanent.
The result may be an expensive LED or (possibly greatly) reduced laser
emission.

I accidentally blew one visible laser diode by neglecting to monitor the
current but it wasn't the sudden effect some people describe - the current
really had to be cranked up well beyond the point where the brightness of the
laser beam stopped increasing. It did indeed turn into a poor excuse for an
LED. One data point and you can conclude the world. :-)

Another one was blown by assuming that a particular driver circuit would work
over a range of input voltages when in fact it was supposed to be powered from
a regulated source. At first the degradation in brightness appeared to be
reversible. However, what was probably happening was that damage to the laser
diode was occurring as soon as the brightness appeared to level off. The
natural tendency was then to back off and approach this same point again. Not
quite as bright? Crank up the current. Finally, once it is much too late,
the realization sets in that it will *never* be quite as bright as it was
originally - ever again. This one still lases but at about 1/10th of its
former brightness.

If you then try to power this damaged laser diode with a driver circuit using
optical feedback, further instantaneous damage will occur as the driver
attempts to maintain the normal optical output - which is now impossible to
achieve and only succeeds in totally frying the device as it increases the
current in a futile attempt to compensate.

And a comment about the expensive Nichia violet laser diodes (see the section:
Availability of Green, Blue, and Violet Laser
Diodes). Physically, they look like ordinary laser diodes and except for
a higher voltage drop, the driving characteristics are basically similar.
However, I've heard that they are even more sensitive to EVERYTHING than
their visible and IR cousins and will degrade or die more easily. Since
the wavelength of these diodes (in the 400 to 420 nm range) is basically
useless for applications requiring visibility, aside from the
"being the first kid on your block" factor, I'd stay away from them until
the price comes down dramatically! I suspect that the newest 430 to 445 nm
Nichia diodes are equally tempermental.

Alternatives - Diode Laser Modules, Laser Pointers

Where what you really want is a working visible diode laser, a commercial
laser pointer or diode laser module may be the best option. Both of these
include the driver circuit and will run off of unregulated low voltage DC.
While the cost may be somewhat higher than that of a bare laser diode, the
much reduced risk of blowout and built-in optics may be well worth the added
cost. It doesn't take too many fried laser diodes to make up this cost
difference!

Believe me, it can get to be really frustrating very quickly blowing expensive
laser diodes especially if you don't really know why they failed. This will
be particularly true where the specifications of the laser diode and/or driver
circuit are not entirely known - as is often the case. Helium-neon lasers are
much more forgiving!

Buy one that accepts an unregulated input voltage. Otherwise, you can still
have problems even if you run the device from a regulated power supply. All
laser pointers and most (but not all) modules will be of this type. However,
if you get a deal that is too good to be true, corners may have been cut. A
proper drive circuit will be more than a resistor and a couple of capacitors!

To confirm that the driver is regulating, start with an input near the bottom
of the claimed voltage range and increase it slowly. The brightness of your
laser diode should be rock solid. If it continues to increase even within the
supposedly acceptable range of input voltage, something is wrong with either
the laser diode (it is incompatible with the driver or damaged) or driver (it
actually requires a regulated input or is incorrectly set up for the laser
diode you are using). Stop right here and rectify the situation before you
blow (yet another) laser diode!

See the chapter: Laser and Parts Sources for
a number of suppliers of both diode laser pointers and diode laser modules.

If you still aren't convinced that someone else should deal with laser diode
drive design issues, the remainder of this chapter provides suggestions for
integrated drive chips, sample circuits, and complete power supply schematics.
But don't complain that you haven't been warned of the sensitive nature of
laser diodes.

The following four possibilities exist for the laser diode drivers inside
laser pointers. (Unless otherwise noted, this applies to red laser pointers,
not the DPSS green types with their high power laser diode pump requirements.)

Series resistor: There is no active regulator. A resistor
limits current to a safe value with a fresh set of batteries. The laser
diode is driven like an LED. As the batteries are drained, current decreases
proportional to the difference between the battery voltage and the diode drop
(about 2 V) divided by the resistances. Since output power and thus
brightness would also decline dramatically with battery use, this approach
is only found in the cheapest of laser pointers. See the section:
Laser Pointer with a Resistor for a
Regulator.

Constant current: Laser diode current is set to a safe value
between threshold and maximum. This takes care of battery voltage variations
but still would have problems with changes in the laser diode output with
temperature. This is rarely, if ever, found on red laser pointers but is
used for green laser pointers since the high power pump diodes for the DPSS
laser module do not have or need optical feedback for adequate regulation.

Optical feedback - unregulated reference: Some laser diode drivers
use the monitor photodiode to control laser diode current but do not have
constant voltage source like a zener diode circuit to use as a reference.
This is fairly safe for the laser diode as long as the correct battery types
are used. For these, output brightness will vary somewhat with battery
voltage and will thus decline as the batteries are drained.

Optical feedback - regulated reference: The best designs (and all
those using IC driver chips) will maintain nearly constant output power until
the batteries are nearly exhausted.

I'd expect to only see (3) and (4) in modern red laser pointers with (4)
predominating in more modern designs. Expect (2) in green DPSS laser
pointers (but many or most of these will also be pulsed).

Visible laser diodes generally have very precise drive requirements. Too
little current and they don't lase; too much current and they quickly
turn into poor imitations of LEDs or die entirely. At least that's true of
most of them. In order for a simple resistor to set the current precisely
enough, it would have to be selected for each laser diode to limit the
current to a safe value with fresh batteries over the expected temperature
range. With only 5 to 10 percent between lasing threshold and maximum
current for a typical visible laser diode, this could be impossible. Until
recently, I had heard that this type of design (or lack thereof) has been
used but had never seen such a simple circuit in a laser pointer.
Apparently, visible laser diodes are now mass produced with a much larger
range of current between threshold and operating limits - possibly engineered
specifically for the ultra-cheap laser pointer market.

(From gabbardo@cpovo.net.)

Well, I have in my hands a laser pointer that has only a resistor to limit
the current instead of the transistorized circuits usually found. It have a
51 ohm SMD type resistor on the PCB in series with the power switch, the
laser diode, and 3 LR44 batteries (1.5 V each).

In fact, the laser diode has no monitor photodiode at all - it have only
2 terminals. The metal case is open on the rear, so one can easily see the
laser diode itself inside it. Interesting enough is that it is the only
type of laser pointer that I can actually now find here (Brazil), but some
years ago I bought some pointers having a complete regulator circuit.

(From: Sam.)

He's has sent me a sample, all the way from Brazil! Heck, it arrived faster
than some of the stuff I send next door. :) As advertised, it certainly
appears not to have anything inside other than a laser diode chip on a heat
sink, 51 ohm surface mount resistor, on-off switch, and battery.

I have measured the I-V curve for both the overall circuit and just the
laser diode. It is consistent with a 51 ohm series resistor and 20 ohm diode
resistance with about a 2 V drop at just above 0 mA (the knee of the diode
I-V curve). The threshold is around 15 mA and the operating current is 35 mA
at 4.5 V (the normal battery voltage) - a rather wide range for a visible edge
emitting diode. My hypothesis is that these laser diodes are specifically
designed to have a wide operating range - possibly by reducing the
reflectance of the output facet and thus the gain, possibly by varying the
doping, or something else. So, efficiency is lower but with the benefit of
increased tolerance to power supply current variation (though 35 mA for
a few mW of output power is a very respectable value).

Someone else sent me a similar pointer and while I haven't actually measured
its I-V curve, I expect that it behaves basically the same. These are both
bullet-style pointers of obviously really cheap construction that came with
5 screw-in pattern heads (1 clear and 4 HOEs). Another better quality
bullet-style pointer I have uses the normal laser diode in a can package
with a regulated driver.

I also bought a couple dozen as-is pointers in a single lot on eBay which are
all of this type.

My general recommendation would be to avoid this if possible but I do agree
that having to spend huge $$$$ for those silly button cells can get to be
annoying. :)

"My laser pointer requires those little button cells which are really
expensive and hard to find. I was wondering if I can instead connect 2 wires
and make a battery pack for it using 3 AA batteries. Do all pointers have
power regulators?"

They all have some sort of regulation but it may not be adequate to deal with
much of a change. You would have to check circuit to be sure or use batteries
that are exactly the same maximum voltage. Even that isn't totally guaranteed
as really dreadful designs could depend on the internal resistance of the
batteries to limit current. So, replacing AAA Alkalines with D Alkalines
could cause problems with some designs.

To be reasonably safe, you would have to measure the current using a fresh set
of the recommended button cells and then add enough series resistance to make
sure the current can never exceed this value even with brand new AAs (or
whatever you are using).

Note that the much more complex and expensive green laser pointers should have
decent regulation but they may still assume that nicely behaved batteries are
used. Therefore, if adding an external power source to one of these, it is
best to make sure it is well filtered, regulated, and has absolutely no
overshoot during power cycling. Also see the next section.

Unlike high quality and expensive diode laser modules, laser pointers may have
less than stellar internal regulation. Thus, you could easily destroy them
instantly by attaching an external power supply, wall adapter, or
even a higher capacity battery of the same voltage as the one used originally.
Some pointers may even depend on the internal voltage drop inside the
recommended (internal) batteries to provide some of the current regulation!

So, if you really want to run a pointer from an external source, the best
thing to do would be to measure the voltage across a fresh set of batteries
powering the pointer and build a highly filtered, well regulated power
supply to match it. The power supply must have absolutely no overshoot or
undershoot when power cycling.

Another not quite as robust alternative is to obtain a wall adapter with an
adequate current rating and slightly higher voltage rating than the pointer's
battery. Then, add series resistance until the voltage at the pointer is the
same as when powered with its internal battery. This is risky, however, since
unless the wall adapter is regulated (few are), ripple, line voltage
fluctuations, and power surges will get through it - and any of these can
fry a laser diode in next to zero time.

Also note that a fancy regulated power adapter may actually be deadly to
a laser pointer. Power supplies that include active components (those using
switchmode or linear regulators as opposed to simple wall adapters with only
a transformer, rectifier, and filter capacitor) may produce sub-microsecond
(or longer) overvoltage spikes when power cycled (at power-on or power-off).
These will have no effect on most electronic equipment but may be fatal to
laser diodes.

As far as connecting the power supply: If you don't mind drilling a hole in
the case or end-cap, construct a dummy battery with contacts at each end
which you wire to your external power supply. Drill a hole in the side of
the case, or better yet in the cap (but off to one side so the cap will still
make proper contact with the battery if you decide to use the pointer with a
battery in the future) to allow the pair of wires to pass through after the
cap is screwed on. There are all sorts of ways of doing this. The
connections have to be made to the center spring contact on the circuit board
at the bottom of the battery compartment and the case. Make sure you get the
polarity correct!

With the wide availability of inexpensive laser pointers in particular, it
would be nice if there were a way to make them do something more exciting than
just project a steady red dot.

Typical questions go something like:

"Hi is there any way I can make my laser pointer blink at an adjustable rate,
something that will turn on/of maybe with the control of a adjustable
resistor? Are there any schematics or something to help me out?"

In principle, a simple circuit based on a 555 timer, for example, could be
used to control power to the pointer or module - perhaps even just control a
relay to act as the on/off button.

In practice, whether this will work or not depends on the design of your
laser pointer or diode laser module. Some have significant filtering and
delays circuits inside which will make blinking at a useful rate impossible.
Others will work fine. Still others will fail due to the repeated stress of
on/off cycles.

Going any deeper into the circuitry than the batteries/power supply or on/off
switch is definitely not for the beginner - if possible at all. Unfortunately,
however, that may be necessary to achieve a useful result. For more info, see
the sections of this chapter on laser diode power requirements, modulation, and
the sample laser diode driver schematics.

(From: Peter Pan (peterpan@cheerful.com).)

Yes! I've used a simple 555 timer circuit driving an emitter follower
transistor buffer amp, to drive several laser pointers. I've had little
trouble recovering a near square wave at the receiving end with a
phototransistor driven amplifier, up to about 5 kHz. After that, the
residual energy stored in the laser module's driver circuit starts to
degrade the square wave, but this can usually be extended, at least
through the remainder of the audio range, by using a push-pull or
complementary-symmetry type buffer, instead of a simple emitter follower.
If you need to go beyond 4 kHz though, it is better to attempt to modulate
the intensity rather then try to accomplish complete shut down/turn on.

Integrated Circuits for Driving Laser Diodes

Many semiconductor manufacturers offer laser driver chips. Some of these
support high bit rate modulation in addition to providing the constant current
optically stabilized power supply. Other types of chips including linear
and switching regulators can be easily adapted to laser diode applications
in many cases. However, some of these chips are designed in such a way that
they will work only at the high bit rates advertised maintaining a continuous
carrier at all times or with a 50 percent average duty cycle or something
equally annoying if all you want is a CW laser diode power supply or even one
for low bit rate communications. You need to check the specs very carefully
for non-standard (e.g., not covered in the datasheet or app-note) applications.

Note: Free samples of ICs like laser diode drivers may be available for
the asking even if you won't be buying a million parts in the future.
Manufacturers often provide some means of requesting free samples at their
web sites. Just be honest about your needs - they consider it good PR and
you might just tell a friend or colleague who WILL buy a million parts!

Analog Devices
(http://www.analog.com/) has several laser diode drivers including the
AD9660
which provides for full current control using the photodiode for
feedback and permits high speed modulation between two power levels.

We are using the OPA 2662 (Burr-Brown) for this. It is an OTA with 370MHz BW,
59 mA/ns SR, and can source/sink 75mA of current per channel (two channels per
chip which may be paralleled quite easily). The part provides the emitter of
the current source to an external pin (programming side of an internal
current mirror), so that a single resistor sets the voltage-current transfer
characteristic. Watch out for the dependence of the harmonic distortion specs
upon the supplied current and frequency though...if this will be a problem
for your particular application that is (didn't matter much for mine).

Elantec (http://www.elantec.com)
offerings include the EL6251C and EL6258C which provide laser diode driver
and sense circuitry. They support high speed control of laser diode current
with selectable levels for read and write, optical feedback regulation, and
protection from low power supply or open input conditions. These parts are
intended for CD, CD-R, CD-RW, and other optical data storage applications.

Another chip, the EL6270C, features an integrated high frequency modulator
(HFM) oscillator to provide output current drive of up to 100 mA, an
external resistor that controls the average laser diode output power, and a
low power disable mode that powers down to 5 uA.

Complete datasheets are available at the Elantec Web site.

(From: Alan Wolke (kc2bog@worldnet.att.net).)

Check out the datasheets for several laser driver circuits available on the
market for high speed fiber communications. See Maxim, HP, Sony, Philips,
Fujitsu, Microcosm, etc. Also, there are many papers in Bell System
Technical Journals that deal with other bias control schemes that don't
involve optical feedback.

iC-Haus Corporation
(http://www.ichauscorp.com/)
offers several CW laser diode driver and controller chips. The complete
datasheets are available on-line and include functional block diagrams and
application information. These devices require only a few common external
components and can be used for CW and modulation/pulsed operation up to
several hundred kHz (depending on model).
iC-Haus parts are available through electronics distributors.
In addition to other information, there is a new White Paper on the design
and test of fast laser diode drivers at
http://www.ichaus.biz/wp4_fastlaserdriver or
http://www.ichaus.biz/upload/pdf/DesignTestFastLaserDriverWP4en.pdf.

App Note AN52 (and probably others) includes a sample circuit using their
one of their chips (not necessary dedicated laser drivers) for powering
laser diodes. In AN52, the LT1110 Micropower DC-DC converter is used as
the current regulator for operating from a 1.5 V battery. However, it is
possible that behavior at low battery voltages might be undefined - and bad
for the laser diode! I wonder if they tested for that? :(

There is an article in the November CQ magazine by WA2NDM entitled, "A
Laser Diode Transmitter" which is based on AN52. However, his circuit uses
an audio transformer to directly modulate the laser diode current and it
would seem that without some additional protection, if someone were to
accidentally drop or tap on the microphone - or power cycle the preamp -
poof goes the laser diode! :(

Laser Diode Power Supply 3 (RE-LD3) uses
a similar chip - the LT1054 DC-DC Converter, not for voltage stepup but to
very effectively isolate the laser diode from input voltage spikes.

The Maxim Engineering Journal (a monthly or so publication you will receive
if you have requested their CDROM and possibly included in trade rags like
EDN and Electronic Design) sometimes has laser diode related articles. For
example, the Special Fiber Optic Edition (early 1999) is devoted to
applications of Maxim's high speed (622 Mbps and up!) optical interface
components including laser diode drivers and sensors. (The Maxim application
note Driving a Laser
diode at 622 Mbps From a Single +3.3V Power Supply may be one of those
from this publication.) The next issue I received, Volume 33, included a
circuit similar to the one described in
Digitally Controlled Laser Diode Driver.

Both Sharp and
Mitsubishi manufacture IC's for
driving laser diodes. Most will maintain constant power. Some require two
voltages, others just one. These circuits will drive the common cathode
lasers, or the Sharp "P" or the Mitsubishi "R" configuration which has the
laser's cathode connected the the anode of the photo diode. The Sharp IR3C07
is a good for CW or analog modulation, and the IR3C08 or IR3C09 will allow
digital modulation to 10 MHz. These parts are quite inexpensive.

Some additional manufacturers of laser diode driver chips and modules include
(this not a complete list!):

The bottom line is that these should be fine for CW laser lights and laser
pointer type applications but NOT for modulation as may be claimed by the
distributors of these modules.

(From: Art Allen, KY1K (aballen@colby.edu).)

I called a person I know who works for a major surplus house. He asked NOT
to be identified. He did give me valuable information regarding the NS102
laser driver modules that are being sold for $3 each (in large quantities) on
the internet.

Here's what I was told.

The NS102 is mass produced in Asia. The chip that the NS102 PCB is based on
is unknown, and probably made in Taiwan too. There are no specs for it.
Only DC parameters are given on the 'rough spec' sheet (advertising quality
literature) the sellers give you.

They do work and they work well.

They use low power and they are stable-if the voltage in changes from 4v to
8v, the LD output remains fairly constant.

However, they are NOT suitable for modulation of laser diodes and should
only be used as a laser pen power supply!

I have an email from a vendor here which sparked all this speculation
regarding their suitability for our purposes. The email CLAIMS they can be
driven to 12 Mhz output pulses while maintaining FULL APC (not average
output monitoring as they do in fiber optic drivers). As far as I can tell,
this is just plain a lie and no one should purchase these expecting to
modulate a laser diode for communications purposes.

They are probably little more than the standard 2 transistor laser driver
that can be used for a laser pointer because it is heavily bypassed with a
heavy duty slow start ramp up circuit.

Some vendors are now selling these for $20 in small quantities - don't get
taken in - it's a laser pointer driver and NOTHING MORE.

If anyone has better info or has tested one of these on the bench, please
let me know. I'd really like to get info on the chip contained on the PCB too.

The NVG laser driver circuit was originally designed for CW only. While I did
not design the driver circuit, I was able to find a way to get it to modulate
successfully up to 2 MHz. I have successfully built a free-space FM modulated
data/voice transmission system using the NVG laser modules (diode, driver,
collimator, enclosure) already set and burned in).

In addition I have helped a number of customers from around the world (Spain,
Italy, Switzerland and the US) use the NVG modules in a modulated design.

While the NS102 type driver circuit does have a 0.1 uF capacitor to act as a
'soft on'/filter protection of the laser diode, by providing enough voltage
to keep the module/laser just below the threshold, you can modulate the NVG
modules (or any suitable diode attached to the NS102 driver) up to 2 MHz. At
that point, it seems that the capacitor effectively filters the modulation and
the circuit 'saturates' and only produces CW output.

Another strategy is to 'inverse' modulate the module - that is, keep the
module effectively on with the modulation signal causing a decrease in power
- rather than have the laser off with modulation causing an increase in
signal....

As far as modulation is concerned, the Analog Devices driver is hard to beat
for three bucks. Couple that with a 555 and a battle proven LM317 front end
and cry 'BINGO'. Maxim used PECL inputs ... arrgh! I don't need to spit photon
packets at 150 mhz! Linear Tech IR receiver looks good, although the $7.00
price tag + a handful of linear doesn't really appeal to me. Too bad you can't
get inside the Epoxy covered die in the Sharp TV/VCR consumer IR receiver
modules (apx $1.50/100 pcs). Not everyone in the world wants to decode bursts
of 40 kHz back into data!

Oh, by the way - an Optek BP812 Optologic sensor performs quite well at at 760
nm. It's an active device available in either totem pole or open collector
outputs. The applications guy at Optek says the device won't work at 760 but
looking at the response curve, I disagree. It's response is only down about
10% in the reds! Most silicon photo stuff is down about 60-75% at 760ish
nm. From what I have seen, the device is very usable at 760 nm. Useful part
for red diodes and HeNe stuff.

More on Laser Diode Characteristics and Drivers

This is getting a little scary. Laser diodes have been around for a good
few years now, and I thought it was fairly widely known how you make
them go and (harder) keep going for a long time; but there have been
several postings recently from folk who are busy making themselves
poorer by driving lasers inappropriately. Here are the rules on how you do it
right:

Just because it isn't hot doesn't mean you didn't already fry it.

Unlike most other things, running them at the "typical" data sheet values
won't work. I'm not talking suboptimal here; I mean that it won't work, not
even a little bit.

You must never, never, never exceed the full rated *optical* power output
of the laser, not even for a fraction of a microsecond. If you do, your
laser will be degraded or dead. This means LOTS of careful design to avoid
nasty switch-on and switch-off transients, for example.

Use the built-in monitor photodiode to regulate the light output. This
monitor diode looks at the leaked light from the back facet of the laser (a
few percent of the useful front-facet output). The current through it is
nicely proportional to light output, if you have a reasonable reverse bias
voltage on it. Anything from 2V to 15V reverse bias is usually OK (on the
photodiode; *never* reverse bias a laser diode!)

The basic problem comes from the characteristics of the laser device. They are
a bit like LEDs, so you will see a forward voltage of about 2.2V for almost
any reasonable forward current (just like an LED, but the voltage is somewhat
higher). Voltage drive is therefore an exceedingly bad idea. Current drive is
a bit more predictable. Up to a certain current - the laser threshold current
- you will get the device acting like a feeble LED. Above the threshold
current, laser operation starts properly and the light output rises very
rapidly as a function of current. Something like this:

The snag is, the difference between threshold and maximum current is usually
quite small; no more than 10% or 20% of the threshold. The threshold current
varies greatly from one device to another (even within the same type number)
and also varies with temperature. Result: setting a fixed current value is
doomed to failure. For some lasers, and on some days, it will be under the
threshold and no laser action will occur; on other days, it will be over the
maximum current and your precious laser will turn into a useless LED (like the
original posting in this thread). The only safe way is to use the monitor
diode current to servo the light output. Even this isn't ideal because the
monitor current is different for different lasers, but:

It doesn't vary significantly with temperature;

Many laser manufacturers give you a test sheet with each device stating the
actual monitor current for full output;

You can provide an adjustment anyway.

But BE CAREFUL. Transient overdriving, even for very short times, can
seriously damage the lasers. Transients commonly occur:

because your feedback circuit rings (or worse. oscillates) so that the
drive current occasionally exceeds the maximum.

because of PSU on/off transients.

because you have used a socket for the laser, and the photodiode connection
is flaky: if it comes disconnected, your feedback circuit will think there
isn't enough drive to the laser and will crank up the current to destruction
level.

because you are trying to modulate the laser brightness with some AC signal
and either you overdo it, or the feedback circuit overshoots.

because you have a pot. somewhere in the circuit to adjust for full output,
and its wiper is noisy.

Above all, remember that it is excessive light output that destroys lasers.
The heating effect of the drive current is not a big problem except that it
has the effect of pushing the threshold current down. Excessive light levels,
on the other hand, can damage the tiny end mirrors of the lasing crystal.

Sharp (one of the big suppliers of laser diodes) also make some nifty 8-pin
drive chips that are pretty good if you don't need to modulate the laser
rapidly. For modulation, consider setting the light output close to 50% of
full output using a really slooooowww-responding feedback circuit, and then
impressing a fixed-amplitude modulating current on the laser. This is OK
because the gradient of the light/current graph is reasonably predictable for
any given laser type, so it's possible to calculate a suitable safe modulating
current from the data sheet.

Good luck to all - and don't forget the eye safety regulations.

(From: Paul Mathews (optoeng@whidbey.com).)

Laser diode structures are usually so small that damage thresholds are very
low on every dimension. The general approach to protecting them is to series
AND shunt filter (and/or clamp) supply voltages to limit the voltage
compliance of current source driving circuits. Also, consider having some of
the current limiting be by means of an actual resistor rather than just active
circuitry. The parasitic capacitances in active driving circuitry can
interact with dv/dt on supply lines to turn on the drive circuit (e.g., drain
to gate capacitance with MOSFET drive), so the resistor limits current even
when this happens. Using bypass capacitance local to the pulse current loop
has the dual benefit of absorbing residual transients and avoiding any effects
of upstream series filter components on speed.

(From: Mark W. Lund (lundm@physc2.edu).

You can blow out the laser in nanoseconds if there is enough voltage and/or
power in the pulse. Two methods: electrostatic discharge type damage which
punches holes in the cavity; brief high power which damages the front facet.

Make sure that the power supply to the modulation circuit is filtered to
prevent surges, isolate the signal circuit to prevent surges on the input line
from getting to the laser.

There are an infinite number of ways to get a damaging pulse. Most common is
the power supply. It helps to have a scope capable of capturing transients
for this. The other ones that I will admit to: using a circuit that wasn't
grounded to the metal optical table--brushing the table with one line of the
circuit and oops; a commercial laser diode power supply that was clean until
we used it in computer control mode when it sent out very fast (anhard to see)
spikes; hooking the laser up backwards; using a power supply that had a big
capacitor across the output which had enough charge in it to do damage; and
forgetting to put a peltier cooled laser on a heat sink (the more current I
gave the cooler the hotter the laser got....oops.)

Well, that was embarrassing, but I hope it encourages others to save a few
(laser diode) lives.

(From: K. Meehan (meehan@srvr.third-wave.com).)

Semiconductor lasers are very sensitive to power spikes. The level of current
that is a problem depends on the laser structure and how much of the current
is converted into optical power vs. heat. In general, reverse current spikes
are very damaging, no matter what level. Make sure that you are modulating
the diode so that you go below laser threshold but not below 0V. In the
forward direction, very short overshoots (<1microseconds) in current can be
handled until you blow the facet off of the device (catastrophic optical damage
- COD). Longer pulse overshoots aren't any better. The current level that
damage occurs varies from device to device. I tend to recommend less than 10%
overshoot in all cases. COD is very easy to note, just look at the laser
(while it is not operating) under a microscope. The facet coating is damaged
near the emission region, if there is a coating. Otherwise, you will see an
enhanced region (darker area) when looking under Nomarski - maybe not so easy
to see.

Another problem that you might be having is spiking during start-up or
shut-down of the device. Current supplies that look lovely during operation
sometimes have spikes in the output when you turn them on or off. You might
want to short the device, making sure that there is no bounce during the
shorting, before turning your supply on or off. There are several laser diode
driver companies out there that make current generators with slow starts and
minimal overshoots. Avtech, Melles Griot, ILX Lightwave, WAvelength
Electronics, etc.

It would be nice if the monitor photodiodes associated with all laser diodes
had the same sensitivity - or even were consistent for a given model. But,
unfortunately, this is not the case.

"I am designing a driver circuit for a laser diode (NEC NDL3220S). The problem
is that the spec sheet says the output of the monitor photodiode at rated
power is max: 0.5 ma, typical: 0.3 ma, min 0.1 ma, at 5 V. This is a huge
range! If I set for 0.3 ma and the actual output is 0.1 mA I will burn out
the laser. I do not have equipment for calibrating the laser output
directly."

(From: Alan Wolke (74150.451@CompuServe.COM).)

Welcome to the wonderful world of laser diodes! You'll find that a 5:1 range
in monitor current is typical, with even a full order of magnitude being
common! This is one reason why most laser diode based applications have a
provision for trimming/tuning the driver circuit to the particular laser.

Your safest bet is to design the feedback loop to operate with less than the
minimum monitor current, and provide the ability to actively tune it to the
appropriate operating point. Thankfully, the relationship between output
power and monitor current will remain reasonably constant over the lifetime of
each particular device. So, once it is properly set, you're done.

The laser diode start time is greatly increased if the LD starts from zero
rather than an LED-level current flow. Wish I'd seen this two years ago!

(From: John, K3PGP (k3pgp@qsl.net).)

For high speed data and very high frequency RF subcarrier/video work I've
always biased my laser diodes to 1/2 laser power then modulated them near
100%, much the same as a standard AM radio transmitter. This does result in
a faster response time rather than cutting the LD completely off. It's also
probably a bit easier on the laser diode especially if it's a high power
unit. (Mine draws 1 amp when putting out 500 mw.)

I never tried biasing it down to BELOW laser threshold at the 'LED' level.
Although this would be an improvement over cutting it off completely, I
would think this would be slower than biasing to 1/2 laser power.

When adding (or removing) external optics, reflections back into the laser
diode itself must be taken into consideration. These can have two effects:

Altering the amount of light hitting the monitor photodiode inside the
laser diode package. This will change the power level setting if the APC
(Automatic Power Control) circuit is being used (as it should be in most
cases).

Destabilizing the lasing process due to reflected light entering the laser
cavity. This effect actually may be more common with low power laser diodes
than one would think. See the section: Causes
of Laser Pointer Output Power Changing When Directed at a Mirror.
However, where the behavior is repeatable and stable, I'd be more inclined
to believe it is the simpler explanation, above.

Note that the losses in the optics are usually only a minor factor where the
power decreases. Even uncoated surfaces reflect only about 4 percent so if
you are getting a 30 percent decrease in power, this probably isn't the cause!

CAUTION: If you remove the optics from a diode laser module, the power may
increase resulting in laser diode destruction, especially if the unit is being
run near its maximum ratings.

(From: Frank DeFreitas (director@holoworld.com).)

The information sheet for a Power Techologies 35 mW module states in bold
capital letters not to even ADJUST the collimation while the diode is running
at full power!

I've got a little 10 mW, 635 nm diode that I tested with and without optics.
Here are the initial readings:

Without Collimating Optics: 10.8 mW.

With Collimating Optics: 10.5 mW.

(I actually expected more of a drop here.)

It is interesting to note that the second reading WITHOUT optics was
3.8 mW and the third reading 2.6 mW. The barrel was becoming very hot. I
killed the power before I killed the diode (I'm learning!). So this
particular diode (from NVG, Inc.) obviously was set up with the collimating
optics in place NEEDS the feedback (reflection) for the photodiode to control
the current.

There is no law that says the internal monitor photodiode must be used in the
driver optical feedback circuit. For some applications, it is desirable to
substitute an external one or use both together. This could be used to control
beam power based on some mechanical condition like position or angle or to
compensate for variations in the behavior of the external optics.

You can't modify a sealed diode laser module in this manner unless it already
has a modulation input but if you are building something from components, it
should be possible. Loop stability must take into account optical path delays
if the distance between the laser diode and photodiode is significant but this
shouldn't be a problem unless you are also trying to modulate the thing at a
very high rate. Obviously, any such scheme must assure that the external
photodiode always intercepts enough of the beam and/or that a hard limit is
imposed by feedback from the *internal* monitor photodiode to assure that the
laser diode specifications are not exceeded under any conditions. Otherwise,
even an errant dust particle or house fly wondering into the portion of the
beam path used for feedback could ruin your laser diode!

Laser diodes in the several hundred mW to multi-watt range which do not have
internal monitor photodiodes have a different set of issues with respect to
safe (for the laser diode, that is) drive circuits.

The dire warnings about instant destruction from overcurrent still apply but
but the extreme non-linearity typical of low power laser diodes isn't usually
present with higher power devices. There is still a lasing threshold but
above this, the output power increases linearly with current and there is
likely to be decent consistency from unit to unit. However, proper current
control and temperature compensation (or adequate derating) is still essential.

(From: Art Allen, KY1K (aballen@colby.edu).)

When you get into the 1 amp diodes (or anything over 200 or 300 mw), the
driver becomes less dependent on the laser power feedback PD and many of
these higher powered diodes just don't have the power sensing PD on-board
for this reason.

While the threshold current is still very dependent on the temperature of
the diode, the DIFFERENCE between the max current and the smoke release
current widens a lot - meaning that the larger diodes can be operated fairly
safely without sampling the output and applying variable current based on
the power sensing PD.

The 1 watt diodes that I was trying to buy several years ago had 2 sets of
specs-one at ambient room temperature and the other set for diodes at
actual operating temperatures-the inference being that the preferred driver
needed TEMPERATURE feedback in order to ramp the diode up to operating
temperature.

Note that these diodes were used to drive fiber optic cables where they
operate as an FM transmitter (constant carrier/fixed duty cycle transmit),
so they probably used a time delay circuit to ramp them up to temperature
rather than an actual temperature sensor.

Where the diode (probably) isn't on constantly, it might be necessary to
derate the diodes and operate them just above threshold in order to be safe.

For your high power diodes, you can use a simple constant current driver
(assuming the diode doesn't require PD based power sensing feedback.

The Vishay Siliconix catalog has an
ABSOLUTELY O-U-T-S-T-A-N-D-I-N-G technical description of MOSFET based
constant current source design. You can request the hard copy of the catalog
from their website, make sure you get the full catalog with the ap notes.
(I couldn't find this on the Vishay Web site but it may be:
"AN103 - The FET Constant-Current Source/Limiter". Feeding "Vishay AN103"
to a search engine should return the PDF.)

(From: John, K3PGP (k3pgp@qsl.net).)

I'm presently using the power supply under
Biasing & Modulating Laser
Diodes - Safely ! on my Web site with
a Russian-made 1 watt 810 nm laser diode. The diode looks like one of those
old time big metal (TO-3 ?) transistors but with a hole in the top of it. The
series resistor in this case was made up out of a bunch of parallel
connected 1 watt 33 ohm resistors. I think I ended up with around 10 to 12
in parallel. This allows me to adjust the laser current in small increments
by adding or subtracting from the number of 33 ohm resistors. It also
solved the problem of trying to find the exact value I needed in a high
wattage resistor. (Wattage rating goes up as you parallel resistors,
resistance goes down.)

I ended up feeding half of the 33 ohm resistors from one 7805 voltage
regulator and the other half from a second 7805. Even though one 7805 can
handle one amp of current it began to show signs of thermal drift when
running at this level. By splitting the resistor bank in half each
regulator only needs to supply 1/2 amp.

A 808 nm 500 mW laser diodes are visible but barely. Do NOT be
fooled into thinking it's not really putting out much power. Human eyes
aren't that sensitive to 800 nm radiation BUT you can easily burn a hole
clean through your retina with this much power. If you doubt this, try
focusing your 808 nm 500 mw laser on the black plastic part of a VHS video
cassette and see what it does. When I do this with mine I get instant smoke
and liquid plastic. So, BE CAREFUL especially when focusing this diode
down to a small spot.

When playing around with stuff like this you will notice that color has a
LOT to do with how much energy is absorbed. Aiming the same laser at the
while label on the same cassette resulted in nothing happening. There is a
very important principal to be learned by this experiment. If the white
label isn't absorbing much power from the laser beam then it has to be going
some place else. The answer of course is it's being reflected (scattered)
back from the white surface. Keep this in mind when playing around with
this diode. If you hit something that's even remotely reflective you could
end up with the beam coming right back at you and you might not even be
aware of it since the human eye is not very sensitive to radiation in the
800 nm region.

For communications use you might want to consider expanding the beam. This
will lower the power density and make it a LOT safer if you accidentally get
in the beam. The beam exiting mine is approx. 4 inches in diameter. 500 mw
spread across a 4 inch diameter circle is a LOT less dangerous than 500 mw
focused down to 1 mm in diameter!!!

And remember that a 500 mW 808 nm laser diode needs a GOOD heatsink.
If you notice the power dropping off shortly after you turn the laser on your
heatsink is too small! If you are having problems with this and you don't
have room for a bigger heatsink use a small 12 VDC fan. Try to direct the
air across the heatsink and NOT across the optics!

You can monitor power output with a regular silicon solar cell hooked directly
to a milliamp meter (not a voltmeter!!!). Do NOT use any series
resistor between the solar cell and meter. Expect to see over 100 ma of
current at this power level. I also suggest you expand the beam to make use
of most of the surface of the solar cell. If you focus it down to a small
diameter the power density goes up and you just might burn a hole in the
solar cell! Plus a very narrow diameter beam could easily bounce off the
shiny surface of the solar cell and hit you in the eyes with enough power
density to do some real damage! Watch the angle between the solar cell and
the laser and anticipate where the reflection might fall. You will get the
same power reading no matter what the beam diameter is as long as all the
energy hits the solar cell. You can substitute a white piece of paper to
get some idea of beam diameter but be CAREFUL when doing this!

Treat this laser with respect. Anticipate reflections. Keep people, animals
and airplanes out of it's path and above all THINK before you turn it
on!

Testing Laser Diode Driver Circuits

This is a basic power supply using a pair IC regulators to provide a variable
voltage with adjustable current limit. Rather than combining these functions
a brute force regulator pair is used - one for the voltage and the other for
the current limit.

The idea is to be able to safely test laser diodes or complete drivers with
the ability to limit current initially to a guaranteed safe value until
circuit operation and/or laser diode behavior can be determined. This should
substitute for an expensive lab supply for testing of lower power devices.

The circuit is shown in Sam's Laser Diode Test
Supply 1 (SG-LT1). As drawn, it is suitable for laser diodes requiring
between about 25 and 250 mA. With obvious changes to certain part values, the
same circuit should be usable at up to an amp or more - but I won't be
responsible for any destruction of expensive laser diodes that might result!

More modern lower dropout regulators like the LT1084 can be substituted for
the LM317. For load currents above about 100 mA continuous, heat sinks will
be required on the IC regulators.

The addition of a voltmeter might be desirable though the knob position of
the voltage adjust pot corrected for the voltage drop of the current limit
regulator will probably be good enough

Very Basic Laser Diode Power Supplies

With care, a very basic power supply can be used to safely drive low and
medium power laser diodes.

The supply I have used to test diodes up to about 2 A is very basic
consisting of a Variac, transformer, bridge rectifier, and filter
capacitor with a current limiting resistor. For low power diodes, this
is typically 50 to 250 ohms; for high power diodes, it is 8 ohms, 50
watt. A bleeder resistor assures that the filter capacitors discharge
quickly once power is removed. A built in voltmeter shows the voltage
into the current limiting resistor at all times. Using the equation:
I=(V-2)/R (2 is the estimated voltage drop of the diode, R is the current
limiting resistor) is often close enough.
Adding a shorting relay which required a press of a button to re-enable
when power is applied would further reduce the risk of accidentally
overdriving the diode.

Since there is no active regulation, the output current has some 120 Hz
ripple so the peak current may be slightly higher than the measured
current. Installing a current meter (A or mA as appropriate) would be
more precise but unless running near the maximum specifications of the
diode, isn't really essential.

Batteries are in fact a relatively safe alternative to sophisticated
power supplies if their characteristics are well understood. Since a
properly connected battery can never put out more than its rated voltage
when new or fully charged, and can't produce reverse polarity, all that is
needed is current limiting via a high power resistor. I would still
recommend a 0.1 uF capacitor, 1N4148 reverse protection diode, and 100 ohm
resistor directly across the diode though.

Here are some guidelines:

A new or fully charged battery can have
substantially more voltage than the nominal rating. For example, a new
Alkaline is around 1.57 V, not 1.5 V. A NiCd may start out at 1.3 V
or more when fully charged.

Don't get too greedy and use a battery voltage close to the diode voltage,
include a reasonable size current limiting resistor and use a higher
battery voltage. The internal resistance of NiCd and NiMH batteries is quite
low and should never be depended upon for a significant part of the
current limiting.

CAUTION: There must NOT be any filter capacitance in the power supply after
the current limiting resistor. This is to minimize the chance that a
bad connection to the diode will result in excessive current should such
a capacitor charge to a much higher voltage and then discharge through
the diode without current limiting.

It's fine to trickle charge a battery while it's being used since
regardless of line voltage fluctuations and spikes, not much will happen
to the battery voltage. However, due to the internal resistance of the
battery, fast charging may not isolate the output enough. Better to
implement a double buffering scheme where one battery is being charged
while the other is in use, switching using a relay with an electrolytic
capacitor to hold the voltage for the millisecond or so when the output
is disconnected from either battery.

The voltage of Alkaline batteries drops steadily as they are used
while that of NiCd and NiMH batteries is nearly constant until they
are fully discharged. Without an active regulator, this must be taken
into account.

To vary the current with no active components, a high power rheostat
or selector switch must be used. Make sure it's wired so that intermittent
contact can't result in current spikes.

For example, to drive a typical IR laser diode, a pair of D-size
Alkaline cells can be used in series with a power resistor. For a
1 W (rated) laser diode which has a threshold of 350 mA, voltage
drop of 1.8 V, and slope efficiency of 0.8, an output power from
near 0 mW to 1 W can be selected as follows:

If you do build these or any other circuits for driving a laser diode, test
them first with a combination of visible (or IR) LEDs and one or more silicon
diodes (to simulate the approximate expected voltage drop) and a discrete
photodiode to verify current limited operation. To accommodate the higher
current of laser diodes compared to LEDs, use several identical LEDs in
parallel with small balancing resistors to assure equal current sharing:

Note that the sensitivity of this photodiode to the LED emission will vary
considerably depending on its position and orientation. Tape the photodiode
and one of the LEDs together (sort of like a homemade opto-isolator) to
stabilize and maximize the response.

Where the laser diode current is below 20 or 30 mA, a suitable opto-coupler
could also be used (see below).

Using this 'laser diode simulator', it will really only be possible to confirm
that the laser driver current regulator is functional, not to actually set it
up for your laser diode.

Once the circuit has been debugged, power down, and carefully install the
laser diode. Double check all connections!

Use the guidelines below in both cases (written assuming an actual laser diode
is being used):

Set the power adjustment of the laser driver to minimum (usually maximum
resistance).

If available, use a power supply with both voltage and current limit
adjustments. Then, you can start with the voltage set to 0 and the current
limit set just above the expected laser threshold current (plus the current
drawn by the rest of the circuit - test with no laser diode in place). This
can always be increased later.

Attach a voltmeter between the photodiode (PD) terminal and ground. This
will effectively monitor relative optical power output.

If you have a (separate) current meter, put it in series with the power
supply as well (or provide another means of measuring current).

CAUTION: Use clip leads. Leave the meters in place - do not attempt to
change connections while the circuit is powered as this could result in a
momentary current spike which may damage the laser diode.

Increase input voltage gradually. Once the laser diode starts lasing, the
PD voltage should climb. The circuit should regulate when the PD voltage
approaches the reference: 2.5 minus .7 V in circuits (1)-(3) or .5 Vcc for
circuit (4). Then, the PD voltage and supply current should level off. If
something doesn't behave as expected, shut down and determine why.

Once you are confident that the circuit is operating properly with the
laser diode installed, the output power can be increased modestly. But,
without a laser power meter, DO THIS AT YOUR OWN RISK!

For visible laser diodes, if you have a laser pointer or other visible
diode laser module OF THE SAME WAVELENGTH, A-B brightness comparisons can
be made if the beams are the same diameter. Otherwise, don't push your
luck unless you have a bucketload of laser diodes you can afford to blow!

For IR laser diodes, visible light eyeballs won't work. The tiny red dot
that may be visible from an IR laser diode cannot be used as an accurate
indication of power output.

Laser diodes are generally NOT very forgiving. However, if you take your time
and make sure you understand exactly what is happening at every step along the
way, you and your laser diode will survive to light another day!

It is an LED optoisolator with a PD output stage. The PD is available by
itself (without current amp transistor) or a moderate gain transistor is
available (base/PD, emitter and collector)-so it's very flexible. The
oveerall combination of LED, PD and output transistor has a 17 Mhz
bandwidth rating.

My feeling was that the PD (standalone) should be used as we are trying to
simulate a PD device itself that is normally inside the LD assy.

The goal is to be able to make the simulator have the same PD sensitivity
as the actual LD/PD combination to be used. I think this is doable without
adding a lot of complexity.

It should make a fairly nice little test jig! It could be made with a
series pot to control LED current (until the proper drive level is
available at the PD output). Several LED's could be switched in/out with a
simple DIP switch, I'm thinking about a self powered flashing LED which
could simulate a variable load for testing dynamic response at slow
speeds-which could speak volumes about some driver circuits::>

A quick and dirty audio monitor on the LD current would be neat too-you
wouldn't have to depend on your eyes to tell you if the drive becomes
unstable or drifts up/down.

Schematics of Laser Diode Power Supplies

The first five circuits are from published circuit diagrams or application
notes, or were reverse engineered from actual devices. All use visible laser
diodes though IR types would work with at most minor modifications to biasing
points.

Laser drivers (1) to (3) were from CW laser lights used for positioning in
medical applications. Laser driver (4) was from a UPC bar code scanner.

Errors may have been made in the transcription. The type and specifications
for the laser diode assembly (LD and PD) are unknown.

The available output power of these devices was probably limited to about 1 mW
but the circuits should be suitable for the typical 3 to 5 mW maximum power
visible laser diode (assuming the same polarity of LD and PD or with suitable
modifications for different polarity units).

Of the 5 designs presented below, I would probably recommend "Laser diode
power supply 2" as a simple but solid circuit for general use. It doesn't
require any special chips or other hard to obtain parts. However, I would
add a reverse polarity protection diode (e.g., 1N4002) in series with the
positive input of the power supply.

Toshiba Laser Diode Power Supply (TO-LD1)

The actual laser driver portion of circuits (1) to (3) as well as the one
presented in the section: Sam's Laser Diode
Driver (SG-LD1) is very similar to the basic design provided in a Toshiba
application note named something like: "Example Driving Circuit for TOLD92xx
Series Visible Laser Diodes".
The Toshiba
Laser Diode Driver Schematic was scanned from the application note by
Kent C. Brodie (brodie@fps.mcw.edu) who also provides a
Circuit
Description. The schematic is reproduced in ASCII, below:

Laser Diode Power Supply 1 (RE-LD1)

This is the circuit from a Scanditronix "Diolase 1" laser line generator,
a unit designed for patient positioning in medical diagnostic and treatment
applications like radiation therapy. No, it doesn't actually engrave the
patient but just projects a red line to aid in placing the patient on the
couch and adjusting couch position in relation to semi-indelible ink marks
drawn on the skin surface.

Laser Diode Power Supply 2 (RE-LD2)

This is the circuit from a Scanditronix "Diolase 2" laser line generator,
similar to the Diolase 1 described in the section:
Laser Diode Power Supply 1 (RE-LD1) but
containing a pair of diode laser modules, normally adjusted to produce a
horizontal and vertical line. It appears to be an improved design including
a soft-start (ramp-up) circuit and an inductor in series with the laser diode.
Otherwise, it is virtually identical and will run from a 6 to 9 VDC source.

Since both units were from the same company, I assume that these refinements
were added as a result of reliability problems with the previous design - in
fact, I have recently discovered that the unit from which I traced that
schematic is not as bright as it should be!

Interestingly, there does not appear to be any reverse polarity protection on
the input - I don't know why that would have been removed! C1 and Q1, at
least, would likely let their smoke out if the power supply was connected
backwards. But Jon Singer added it in his redrawn version,
Laser Diode Power Supply Schematic 2 (RE-LD2), (if you
don't like the ASCII schematic below!)

Laser Diode Power Supply 3 (RE-LD3)

This one runs off of a (wall adapter) power supply providing about 8 to 15 V.

It was apparently designed by someone who was totally obsessed with protecting
the laser diode from all outside influences - as one should be but there are
limits. :-) This one goes to extremes as there are 5 levels of protection:

Input C-L-C filter.

Soft start circuit (slow voltage ramp up).

7805 fixed voltage regulator.

LT1054 DC-DC voltage converter.

Optical power based current source.

The first part of the circuit consists of the input filter, soft start circuit,
voltage regulator, and DC-DC voltage converter. Its output should be s super
clean, filtered, despiked, regulated, smoothed, massaged, source of -5 V ;-).

It was not possible to determine the values of L1 and L2 other than to measure
their DC resistance - 4.3 ohms. The LT1054
(Linear Technology) is a 'Switched
Capacitor Voltage Converter with Regulator' running at a 25 kHz switching
frequency. A full datasheet is available at the Web site, above.

The output of Q1 ramps up with a time constant of about 50 ms (R4 charging C9).
This is then regulated by the 7805.

The LT1054 takes the regulated 5 V input and creates a regulated -5 V output.
There is no obvious reason for using this part except the desire to isolate
the laser diode as completely as possible from outside influences. Like the
use of an Uninterruptible Power Source (UPS) to protect computer equipment from
power surges, a DC-DC converter will similarly isolate the laser diode circuit
from any noise or spikes on its input.

Laser Diode Power Supply 4 (RE-LD4)

This more sophisticated (or at least more complicated) driver board uses a dual
op-amp (LM358) chip instead of discrete parts to control a transistor current
source. Due to the relative complexity of this design, and the fact that it
is entirely constructed of itty-bitty surface mount parts, errors or omissions
with respect to both transcription and interpretation are quite possible!

The feedback loop consists of the photodiode (PD, part of D1), a non-inverting
buffer (U2A), the inverting amp/low pass filter (U2B, R9, R11, C2, bandwidth
of about 1 kHz), and emitter following current source (Q1, R13, R14, with a
sensitivity of 36 mA/V) driving the laser diode (LD, part of D1).

Separate DC inputs are shown for the laser diode/photodiode itself (Vcc1) and
the other circuitry (Vcc2). Vcc1 must be a regulated supply as there is no
on-board voltage reference. It appears as though Vcc1 and Vcc2 should be set
equal to one-another though there may have been (external) power sequencing in
the original application. If Vcc1 is less than Vcc2 by more than a volt or
so, the laser diode will be turned off. The input voltage range can be from 5
to 12 VDC though I would recommend running on 5 VDC if possible since this
will minimize power consumption and heat dissipation in the current driver
transistor and other circuitry. This is adequate for laser diodes with an
operating current of up to about 80 mA. For laser diodes with an operating
current greater than this, a slightly higher voltage will be required.

The set-point is at about 1/2 Vcc1 so that the laser diode optical output will
be controlled to maintain photodiode current at: I(PD) = .5 Vcc1 / (R6||R7).
Use this to determine the setting for R7 (SBT, Select By Test, Power Adjust)
for the photodiode in your particular laser diode. Or replace R7 by a low
noise variable resistor and use a laser power meter to set the operating
current. (Hint: Start with the minimum current - maximum resistance).

Optical output will be linear with respect to Vcc1 and inversely proportional
to R6||R7 as long as the laser diode is capable of producing the output power
(and thus photodiode current) determined by the equation, above. Beyond the
upper limit, the laser diode will likely be damaged instantly! Don't push
your luck too far. :-)

For example, with Vcc1 = Vcc2 = 5 VDC, maximum laser diode current will be
limited to about 90 mA. With R7 (SBT) equal to 5.9K, photodiode current will
be .5 mA. For some laser diodes, this is approximately the value for 1 mW of
optical beam power BUT YOURS MAY BE TOTALLY DIFFERENT!

If you then increase Vcc1 = Vcc2 to 10 V or halve the parallel combination of
R6||R7, the output power will double or the laser diode will die in a futile
attempt to achieve the impossible.

A cutoff circuit is provided to disable current to the laser diode as long
as Vcc2 is more than about 1 V greater than Vcc1 or from an external input
logic signal (ground J1-2 to disable). This consists of Q2, Q3, and their
associated resistors. When Q2 is biased on, it turns on Q3 which shorts out
the input to the main current driver, Q1.

The comparator (U1, LM311) would appear to output a signal based on photodiode
current being above a threshold but its true purpose and function is not at
all clear (or there is a mistake in the schematic).

As noted above, there is NO on-board voltage or current reference. Thus, Vcc1
must be a well regulated DC supply with low ripple and noise and NO power-on
overshoot (especially if the laser diode is being run close to its optical
power limit). However, this isn't quite as critical as driving the laser
diode directly since optical output power (photodiode current) and not laser
diode current is the controlled parameter. A power supply using an LM317 or
7805 type IC regulator with a large high quality filter capacitor on its
output (e.g., 100uF, 16V, tantalum, in parallel with a .01uF ceramic)
should be adequate.

Although the original version of this board uses surface mount devices, common
through-hole equivalents are available for all parts and these are labeled on
the schematic. Note: A heat sink is essential for (Q1) where Vcc1 is greater
than 5 VDC - this part gets warm.

Sam's Laser Diode Driver (SG-LD1)

SG-LD1 is an enhanced version of the design described in the section: Laser
Diode Power Supply 2 (RE-LD2) with the addition of bilevel (digital)
modulation as described in the section: "Laser diode modulation". It should
be capable of driving most typical small laser diodes including those found in
CD players and CDROM and other optical drives, and visible laser diodes
similar to those found in laser pointers, bar code scanners, medical
positioning laser lights, and other similar devices.

This design assumes a laser diode assembly where the laser diode anode and
photodiode cathode are common (this seems to be the arrangement used most).
If the opposite is true with your device (laser diode cathode and photodiode
anode are common), reversing the direction of polarized components and power
supply input, and changing NPN transistors to PNPs and vice-versa will permit
the same PCB layout to be used. However, if your laser diode assembly has
both anodes or cathodes in common, this circuit is not suitable unless an
external photodiode is used for the optical feedback.

Disclaimer: The cicuit is currently under development so there may still be
errors in the schematic and/or PCB artwork. I will not be responsible for
any damage to your pocketbook or ego if for some reason your laser diodes
do not survive. (This disclaimer may never go away!)

In some cases, the part values listed should be considered as suggestions as
many modifications are possible depending on your particular laser diode
specifications and application needs. Transistors with heat sinks for Q2 and
Q4 are advised if operating continuously near the upper end of the input
voltage range (say above 10 V) and/or at laser diode currents of 100 mA or
higher.

Input power (Vcc) can be anything in the range of about 10 to 15 VDC. It's
not critical and will have no effect on the output power. A regulated
supply isn't required.

Ebl should normally be left open. A switch closure (or open collector
NPN transistor or open drain MOSFET) to Gnd shuts off the
driver. Do NOT apply any active high signal to this input.

The monitor photodiode current at rated power will be in the specifications
for the laser diode, usually with a rather wide range of sensitivity (10:1
or more). To start out, assume it's the minimum value and then if that
doesn't result in enough output power (or any lasing at all with proper
circuit operation confirmed), reduce the resistor values to obtain the desired
output power. The reference point is a voltage of about 3.2 V on the base
of Q1. For example, if the monitor photodiode current at full power is
0.5 mA, the total resistance would need to be about 6.4K ohms minimum.
However, since the monitor photodiode sensitivity can vary widely, start
with a high enough total resistance so that even worst case, the laser
diode will be safe. Then, reduce the resistance once the behavior has
been determined.

A positive voltage (3 to 15 V) applied to Mod turns on Q3 which shorts out
R7 and increases the output power by an amount determined by the
values of R4, R7, and the setting of R5. The specific resistance values
must be selected based on the desired output power, modulation index, and
monitor photodiode sensitivity.

CAUTION: As with all low power laser diodes, it is essential to use a
laser power meter to determine the setting for maximum power.

A printed circuit board layout is also available. The entire single sided
circuit board is 1.7" x 1.15" and includes modulation and enable inputs. It
will run on an unregulated power supply of around 6 to 12 VDC.

The layout may be viewed as a GIF file (draft quality) as:
sgld1pcb.gif.

The Gerber files include the solder side copper, soldermask, top silkscreen,
optional component side pads, and drill control artwork. The original printed
circuit board CAD files and netlist (in Tango PCB format) are provided so that
the circuit layout can be modified or imported to another system if desired.
The text file 'sgld1.doc' (in sgld1grb.zip) describes the file contents in
more detail.

I have a few bare (unpopulated) PCBs fabbed from this artwork available,
as yet untested.

Modification of SG-LD1 for Common Cathode LD/PD (SG-LD2)

While most laser diode packages have the configuration assumed by all the
previous driver circuits, there are some that don't fit the mold. This
section deals with one variation in particular - those with a common cathode
connection.

A simple modification to the basic SG-LD1 circuit (or any of the others that
are similar) should permit these types of laser diodes to be safety driven.

Sam's Laser Diode Driver 2 shows the new circuit.
The only changes are to the wiring of the laser diode package and the
substitution of a zener diode (CR3) for R8. CR3 guarantees that the laser
diode will not be driven should the voltage on the photodiode be insufficient
for the feedback control to be active. At normal supply voltages, leaving
R8 in as in SG-LD1 should work. The concern is that during power cycling or
if run from a power supply voltage that is too low, the circuit could attempt
to overdrive the laser diode thinking there is inadequate output power due to
lack of bias on the photodiode and/or not enough voltage on the feedback
components.

Sorry, no PCB layout available for this one. Modifications to the SG-LD1 PCB
layout are left as an exercise for the student. :)

K3PGP's Laser Diode Driver (K3-LD1)

This one runs open loop (no optical feedback) but has been designed to permit
safe modulation. It should be fine as long as you don't try to run too close
to the laser diode's maximum current/power rating.

Viacheslav's Laser Diode Driver (VS-LD1)

The circuit in Viacheslav's Laser Diode Driver
(VS-LD1) is quite straightforward. I guess my main nit to pick would be
that it uses more power than needed due to the constant current driver as
opposed to a constant voltage source and a means of controlling the current
via a pass transistor. But for a low power laser diode, this really isn't a
major concern. There is enough filtering on the input that any transient
conditions should not cause problems.

(From: Viacheslav Slavinsky (svo-@-rbcmail.ru).)

I started with a constant current source using a LM317L (DA1) and R1.
The current then branches to laser diode (through R5 for fine adjustment
of division ratio and R6 for monitoring) to KT3 (LD anode). Another
branch on VT1 is made to sink the extra current, the more the feedback,
the more current sinks through the transistor. R2 regulates the reverse
bias of the photodiode (it actually doesn't need to be 20K, but I picked
from what I had in local store).

KT3 is the LD anode, KT4 is the PD cathode.

This circuit looks pretty stable (I can only judge by eye and voltage
meter). For tests I used 2 metal-cased LED's and some unknown
photodiode. Green LEDs could not impress the photodiode so I just used a
laser pointer to check that feedback works. After I was sure that
everything was all right, I set current to about 50 ma and plugged in the
laser diode (Mitsubishi ML1016R, I = 80 mA). Then it was easy to set the
nominal current and test the feedback a little against circumstances
(unattaching it from heatsink for a few seconds, for example).

Actually before this circuit I assembled one similar to SG-LD1,
just altered it to adopt Mitsubishi's pinout. But while testing it I
felt like I'm not 100% sure how it works and I was very paranoid about
LD sensitivity to everything and knew very little practical stuff, so I
decided to make my own circuit. Yes, it indeed draws 120 mA where only
90 mA are used for good, there's room for improvements.

Laser Diode Driver from Red Laser Module 1 (RLM-LD1)

This circuit was found in a 25 mW red laser diode module, model and
manufacturerer unknown. It is almost an exact mirror image (with respect
to polarities) of Toshiba Discrete Laser
Diode Power Supply (TO-LD1). Note that the input voltage is negative.

Laser Diode Driver from Cheap Red Laser Pointer 1 (LP-LD1)

(This circuit was reverse engineered by Jim Moss (Jim.Moss@nsc.com) who also
provided the circuit analysis.)

It is from a cheap laser pointer. Like the other discrete laser diode
drivers, a single PNP transistor is used in the feedback loop to regulate
laser diode current. However, although optical feedback of sorts is used,
there appears to be no real reference. Thus, output power will depend on
battery voltage, nominally 4.5 VDC (3 button cells, I assume) and the gain
of Q2.

At first I thought some parts had been left out: At the very least, a zener or
similar reference across C-E of Q2, and possibly some filter caps to keep the
thing from oscillating. While was willing to believe that the design had
the optical output depending on battery voltage, it seemed inconceivable
for it to be directly affected by the gain of the driver transistor. However,
I now believe that it is probably drawn correctly but the actual operating
point is where the Q1 is almost in cutoff and its gain wouldn't be critical.

Laser Diode Driver from Cheap Red Laser Pointer 2 (LP-LD2)

This is the circuit from another inexpensive laser pointer.
Although very similar, it includes some capacitive filtering (and more
optional filtering in C2, not installed), as well as a power adjust pot (VR1).
However, like the previous circuit, this does not have any absolute reference
so power output will be dependent on the battery voltage to some extent.
People have successfully modulated this module at a reasonable frequency
(upper limit not determined) by removing or greatly reducing the value of the
filter capacitor, C1. However, do this at your own risk!

This unit was available from Oatley
Electronics (AU) as the module LM-2 (January, 2000). Of course, they may
have already switched to a different supplier or the manufacturer may have
changed the design!

Since Ipd is proportional to optical power output, like LP-LD1 and LP-LD2
(above), brightness is dependent on battery voltage. In this case, it is a
much more non-linear relationship as Vld and Vbe1 set a threshold of about 2
to 2.5 V below which there will be nothing and then output will increase based
on Vbatt/(R1 + VR1). The circuit operates on 3 V but 4.5 V seems like the
minimum to get any decent output.

Laser Diode Driver from Cheap Red Laser Pointer 5 (LP-LD5)

This is the circuit from another inexpensive laser pointer. Well,
actually it's from a diode laser module, but this was obviously just a
pointer driver without the pushbutton (which I have added in the
schematic). Battery voltage is 2.6 to 3.0 V. It's very similar
to LP-LP1 and LP-LD2, above.

Laser Diode Driver from IR Laser Module 1 (ILM-LD1)

This is a very simple circuit from a 780 nm laser diode module
sent to me by Shawo Hwa Industrial
Co., Ltd., a Taiwanese manufacturer of laser pointers, laser modules,
and other related laser devices. This unit is similar to the guts from
a typical visible laser pointer. Connections are via wires though there
is a battery contact spring hidden under heatshrink, but no switch or power
adjust pot. The laser diode is in a 5.6 mm metal can though the window
appears to be molded in place rather than glued from the inside.

The battery voltage is spec'd at 3 V. The only reference device is the
B-E junction of Q1 so power output will vary with temperature and not very
much with battery voltage. Both SMT transistors were labeled "RIP". R1
could be changed to a pot to provide a variable power adjustment. I assume
that for this module, its value is selected for each laser diode. I'm not
sure what the rated output power is for this module other than "<5mW"
but it actually measured 2.3 mW.

Laser Diode Driver from Green Laser Pointer 1 (GLP-LD1)

Here is the schematic for the driver from a CW green DPSS laser pointer
generously contributed to the cause by
Laserpointers.co.uk. There
is no model number on the case but it is manufactured by Lightvision
Technologies Corp., Taiwan. The pointer was given to me because (1) it was
broken and (2) Laserpointers.co.uk apparently doesn't deal with this supplier
anymore so they couldn't send it back for repair.

The pointer is in a nice dark blue case with gold and chrome trim. It was
quite dead. However, fiddling with the batteries while completing the contact
from the positive terminal to the case resulted in some flashes of green
light and with just the right pressure, a continuous beam. So, there had
to be a bad connection inside. Clamping the chrome cap on the output-end
in a vice with some protective padding and wiggling resulted in it coming
loose relatively easily. The result is shown in
Components of Typical Green DPSS Laser Pointer.
It turns out that the laser module
consists of several parts. Sorry, no complete dissection. :) These are
screwed together with dabs of glue to keep them from shifting position.
However, the positive return for the battery also goes though these joints
(from rear cap though case to front cap, IR filter holder, collimating lens
holder, DPSS module, laser diode case and finally back to the driver board).
And one of the joints wasn't exactly tight. Perhaps, the path is really
supposed to be via contact between the case and the DPSS module directly
but the lumps of glue prevented this. So, I wrapped some bare wire around
all the parts and then covered this with aluminum foil and tape. ;-)

The circuit in Green Laser Pointer Diode Driver 1
is a basic dual op-amp constant current driver. All part values
were either labeled or measured except for C4 since I didn't risk putting
a capacitance meter across the laser diode. But C4 looks identical to the
others so there is high degree of confidence in the uF value. D1 and C1
provide soft-start and the pointer doesn't seem to mind reverse polarity
(either by design or because Murphy took a day off). All in all, not a bad
little circuit. No, I don't intend to turn the pot. ;-)

The circuit in Green Laser Pointer Diode Driver 2
uses what appears to be a low voltage 33202 dual op-amp. Do a Google search
for "MC33202".) It's configured as a squarewave oscillator feeding a constant
current driver. Part values for the capacitors were all guessed because they
wouldn't produce meaningful readings on either of my DMMs. This is still a
mystery.

Laser Diode Driver from Green Laser Pointer 4 (GLP-LD4)

Here is the schematic for the driver from a
Z-Bolt BTMK-10
green DPSS laser pointer. This one is rated at 5 mW, though I
assume the same design is used for some higher power versions.
The ZBolt BTMK-10 is actually not a pointer in the
usual sense since it doesn't have a momentary switch on the side
and is aimed at (no pun...) targeting applications. But I'll call
it a pointer here. :) The switch is on the rear end and is latching.
This one differs from the 3 previous
drivers in that it uses Automatic Power Control (APC) rather than Automatic
Current Control (ACC, constant current). So, the feedback loop is
closed by a photodiode that samples a portion of the output beam.

The circuit in Green Laser Pointer Diode Driver 4
uses what appears to be a low voltage ELM8548M1 dual op-amp. However,
as can be seen in the schematic, there is no feedback resistor for the
second op-amp so perhaps that has one built-in. The parts were all
labeled, though I'm not positive about which labels went with which
parts in a couple cases. There is also space for a tiny surface mount
LED and its current limiting resistor.

While the APC circuit operation is quite straightforward, there would
seem to be a potential issue should the circuit be incapable of obtaining
the expected output power. Since there is no absolute current limit,
it could drive the laser diode to destruction should someone power it
in a cold environment where the diode wavelength doesn't match up
with the vanadate absorption and it can't produce 5 mW at rated diode
current. The current would then be limited only by circuit
and battery resistance. However, if the designers were really clever,
they might have set up the beam sampler to just enough pump light leaks
through to the photodiode and limit the current even with insufficient
green output. However, I rather doubt this to be the case since there
is no way to adjust any current limit.

Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic
transistor type case) and the LM317T (TO-220 7805-type case), the pins are,
left to right, Adjust-Output-Input.

For the resistor, I use a small carbon 10 ohm in series with a precision
10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms.

On initial power on, use three garden variety diodes stacked in series
instead of the laser diode. Put a current meter in series with the diode
stack and adjust the precision resistor for 50-60 mA. Disconnect power and
replace the diode stack with the laser diode. Connect up power again, still
watching on the current meter. The diode will probably initially glow
dimly. I use a diode that lases at about 72 mA, and has a max rating of 100
mA. I use about 85 mA for normal ops.

Turn up the current, never exceeding your diode's max limit. The dim glow
will increase in intensity, but at some point, a distinctive step in
intensity will occur. Your diode is lasing. Remove the current meter as
desired. Enjoy!

(From: Crow (alias Lostgallifreyan).)

In all the variations of laser diode drivers based on three terminal
regulators like the LM317, there is the
detail of selecting the series resistor. Standard resistor values
rarely match exactly. However, two standard resistors can likely get
you to within 1 mA of desired current on a range up to 5 amps, in a
parallel or series network. But which two will get closest while
remaining under the limit? And how large must they be to handle the
current they'll take? LM317 and LM338 Current
Regulator Resistor Calculator is a C program to find
the best pairing of available resistors for the selected current
at maximum power dissipation. The reasoning behind this is that while
many of those drivers will preset or even modulate a current, many people
will want to do it within a strict, hardwired upper limit, and sometimes
1.25/I just isn't enough information. The code is free to all, and will
likely compile on anything that runs C code in a text-based shell or
console window.

Even though the standard resistor ranges like E24 or E12 only cover
every possible value at low tolerances, metal film resistors are
usually sold as 1% regardless of how small the range is from a
supplier. This means the coverage is like a net instead of a cloth.
Two resistors are chosen by the program on the assumption of infinitely
strict tolerance, but this will still deliver pairings whose current limit is
very close to the desired current in almost all cases. Of course,
the best thing to do is use 1% resistors. If there is a significant
error in some rare case, the comparison of wanted and actual currents
will show it. In this case, try some current to see if there is a
better match to that. There usually won't be because the program tries all
possibilities and gives you its best shot anyway. So use the most
recise resistors you can get and bear in mind that for very low
currents the regulator, rather than the resistors, will determine how
accurate the result will be.

(From: Steve Roberts.)

Here's a similar circuit that will drive pump diodes for solid state lasers
with up to about 0.8 A if a most excellent heatsink is used:

Input power should be regulated 5 to 6 VDC. Since there is some
interaction between diode voltage and current with this design, make sure
to set up the current adjustment with a dummy (e.g., dead) laser diode,
or make sure it is set low before applying power and increase it slowly to
the operating point. Then, fine tweak the current once the temperature of
the diode has stabilized.

EU38 Low Cost Constant Current Laser Diode Driver

This is a small printed circuit board (about 14x35 mm) which will drive laser
diodes in constant current mode up to 800 mA without a heat sink and about
1.2 A with a heatsink (not included). It is suitable for driving laser diodes
not requiring optical feedback such as DPSS laser pump diodes of up to
about 0.5 W output.

The schematic I reverse engineered from the Roithner version can be found
in EU38 Constant Current Laser Diode Driver.
The circuit consists of an NPN power transistor controlled by a single op-amp.
Feedback is taken from a 0.6 ohm series current sense resistor.
One issue that I've found is that the reference is a zener
diode (type unidentified) which probably doesn't have enough current going
through it so while the feedback loop has enough gain and current regulation
is quite good with respect to laser diode characteristics, the reference
voltage changes slightly with input voltage. Thus, I recommend powering
the unit from a regulated supply rather than a cheap wall adapter or
batteries.

Not all components were labeled so it's quite possible there are errors. The
zener voltage was determined by measurement with an input voltage to the board
of about 4 VDC. I'm kind of guessing about the resistance of the Iadj
pot (R4). It's more than 20K and less than 100K, so 50K is a nice standard
intermediate value. The bias current or offset voltage or something :) of
the mediocre op-amp (an LM358 clone) adds about 0.05 A to the output current.

I did find and fix two errors that were in my original schematic: (1) the
value of R6 had been shown as 4.7K rather than 47K and
(2) when I measured the voltage across the zener (ZD1), it was 1.05 V rather
than the 1.5 V I had before. Although I was rather suspicious of that
1.05 V, a similar voltage has been confirmed by someone else.
Perhaps the 1.5 V was wishful thinking when I originally traced the schematic.

The Roithner specs for the EU38 say that it can go to 1.2 A with a heatsink.
As drawn, the maximum current is just about 1 A so there may still be
errors in the schematic. If the resistance of the pot were much higher,
the maximum current might almost get to 1.2 A. Or a user modification may be
needed to go any higher. There are 6 through-pads on the PCB that
I thought might have been intended for this purpose, but 4 are
connected to ground, 1 is connected to power, and 1 is a no-connect.

I have used the EU38 to power the green demo laser described
in the section: Even Simpler Instant Green
DPSS Laser. The complete power supply is shown in
Green Demo Laser Power Supply Using EU38.
One complaint about the EU38 is that a jeweler's screwdriver must be
used to adjust the current and the slot is in the metal wiper of the pot
so it picks up 60 (or 50) Hz noise and modulates the diode current while
touching it if the screwdriver handle isn't insulated!

Super Simple Laser Diode and TEC Driver

Super Simple Laser Diode and TEC Driver
uses a hand-full of Radio Shack parts to provide variable
current to a low power laser diode along with a TEC for cooling.
OK, they will have to be Digikey parts since RS doesn't really sell
parts anymore. :)

The laser diode driver is an adjustable voltage regulator with a
current limiting resistor. Added filtering and reverse polarity
protection guarantee no overshoot or transients when power cycling.
The cooling-only TEC driver is a MOSFET with a pot
for the set-point. With only a MOSFET as the active component, this
won't be very precise for temperature tuning but is adequate to keep
the diode cool. I built it to power a Crystalaser 35 mW red diode laser.
The numbers by LD1, TH1, and TEC1 refer to the 10 pin ribbon cable connector
on the laser head. LED2 provides a rough indication of the voltage across
the TEC, and thus the current through it.

Note that the voltage for the TEC is the same as the voltage for
the laser diode based on the argument that there will be correlation
between the LD power and the required TEC power. It could also come from
the fixed 12 VDC input.

For this low power Crystalaser laser, the TEC is almost unnecessary as the
maximum current to the laser diode is under 100 mA. But it was an excuse to
implement this trivial scheme. In fact, acceptable cooling could be
achieved even without using any active components by simply putting
the laser diode in series with the TEC. But with the MOSFET, it
was somewhat better.

A regulated 12 VDC power supply is recommended. Using a 7812 to provide this
from a 15 to 20 VDC source would be ideal.

There's nothing critical in the circuit. Any sort of common adjustable
regulator can be used. The LT1084 was simply available, but an LM317
would be fine as well. Same for the MOSFET. The BUZ71A just happened
to cry out to be used. :)

CAUTION: This is a more or less constant current driver without optical
feedback. Therefore, it may not be suitable for laser diodes where the
operating range of current is small.

Constant Current Supply for High Power Laser Diodes

(From: Winfield Hill (hill@rowland.org).)

The schematic in the section: Simple Laser Diode Power Supply is the
standard circuit for making a constant current source from an LM317 or LM338
(e.g. see The Art of Electronics, fig 6.38). The problem with this circuit is
that for large currents (the only currents for which it has good accuracy, and
is a serious part saver) it's hard to make the current variable.

For example, for a 3.5 A current source, the resistor value is 0.357 ohms,
and if you then want a 3.1 A current you've got to unsolder it and replace
it with a 0.403 ohm resistor. Bummer.

One option would be to put a low value pot across the sense resistor and
connect its tap to the voltage regulator common/adjust terminal. This will
work reasonably well for a modest current range - perhaps up to 2:1 as shown
below - but runs into difficulties where a wide range of control is desired.

The reason is that this arrangement can only *increase* the current from the
nominal I = 1.25V/R. So, for example, to get a 10:1 range, the voltage across
the sense resistor would be 12.5 V for the 10x current! In general this is not
attractive for the high current condition because not only have you required
a higher supply voltage, at the maximum current, but the power dissipation in
the sense resistor is also quite high (more like HUGE --- sam).

Let me offer the following simple circuit, which I just created and haven't
tried but 'oughta work' as a solution to this problem.

By contrast, this circuit can only *decrease* the current from the 1.25V/R
value, but it easily handles a 10:1 range (or even much more) and the voltage
across the sense resistor is never more than 1.25V, allowing low supply voltage
(e.g. 5 V) and keeping the dissipation low.

The 1K pot selects a portion of the floating 1.23 V reference voltage, and
tricks the LM317 or LM338 into correspondingly reducing the voltage across
the 0.25 ohm current-sense resistor. The pot is conventional and may be
panel mounted. It should be possible to nearly shut off the LM338 (a
minimum quiescent current will still flow). The current sink, I, which
powers the floating 1.23 V reference, is not critical and may be a simple
current mirror (sorry to see the TL011 gone!), or even a resistor to
ground or any available negative voltage, depending upon the desired
current-source voltage-compliance range. That's it!

Sam's High Power Laser Diode Driver 1 (SG-DH1)

This isn't exactly an entire design but one that uses a common logic power
supply in an unconventional way.

It may be possible to use a high current switchmode power supply
as a variable current laser diode driver as long as it has remote
sensing capability. The remote sensing feedback loop maintains a constant
voltage (the spec'd supply voltage) between RS+ and RS-. Normally, this
is used to compensate for the voltage drop in the wiring harness.
By applying a variable control voltage between RS+ and
V+, the power supply can be fooled into producing any output voltage from
near 0 to its maximum rating as long as its minimum load requirement is
satisfied. With a small resistor in series with the
laser diode (or for those willing to take risks, the resistance of the
laser diode), this results in a variable current to the laser diode.
The only limit on output current is the maximum rating of the power supply.
These types of power supplies, capable of 50 A, 100 A, or even
higher current, are readily available on the surplus market. However, this
scheme may only work with certain models, those which power their control
circuitry separately from the main output and don't go into some sort of
undervoltage shutdown if the output voltage goes too low. I don't know
how to determine which models satisfy this requirement.

I have not yet attempted to close the loop and provide actual current
control but have opted for voltage control for now at least.
The unit I've been using for these tests is a Shindengen PS5V100A, a
fully enclosed fan cooled switchmode power supply that's about 15 years
old. This unit is also nice in that it regulates well with no load.
All that was needed was to remove the shorting link between V+ and RS+ and
install a 20 ohm, 2 W resistor in its place. Then applying 0 to +15 VDC
current limited by a 47 ohm, 5 W resistor across RS+ (+) and V+ (-),
the output voltage would vary from near 0 to 5 VDC.

R3 can be constructed from a length of building wire. For example, 20 feet
of #14 copper wire has a resistance of 0.05 ohms but water cooling would be
needed if run near full current. I'm actually only using a head lamp
load for testing and it works fine.

The same scheme using RS- did not have enough range, probably due to the
internal circuit design. This is too bad because the op-amp circuitry to
drive it might have been simpler, or at least more intuitive to design.

(I did try a test of the same approach with a Pioneer Magnetics dual output
power supply (5 VDC at 59 A, 12 VDC at 67 A). While control was possible,
it didn't behave nearly as perfectly as the Shindengen supply. More than
1/2 A of control current was required to change the 5 V output to 4 V. And
while the 12 VDC output could be reduced to near 0 V, the cooling fans
cut out at about 8 VDC so they would need to be powered separately for
continuous operation at high current. But this might be nice for driving
series connected laser diode bars.)

The challenge is to convert this to a user friendly form that is safe for
the laser diode. I am designing a control panel which incorporates
what I hope will be fail-safe circuits to minimize the chance of excessive
current either from power cycling or by user error. It will use closed
loop feedback so the actual current can be set (rather than voltage) and
includes a multifunction panel meter (set current, actual current, diode
voltage). It will enable diode current only if all power supplies are
stable and correct, the 10 turn current adjust pot is at 0, and with the
press of a green button.

However, initially, I'm using a 10 turn pot to control the
current with a digital panel meter monitoring current via a 0.025 ohm
sense resistor. Current is limited to between 50 A by a
0.06 ohm power resistor. Believe it or not, even 50 A is way below the
limit for the diodes I need to test! See the section:
Characteristics of Some Really High Power
IR Diode Lasers.

The basic control panel includes an Enable switch (eventually to be
replaced with a keylock switch), Diode On and Off buttons, the 10 turn
pot and DPM which reads 0 to 100 A. A differential amplifier converts
the voltage across the current sense resistor into a DC voltage for the
DPM. Without the differential amplifier, the control current was seriously
affecting the readings as 1 A is only 2.5 mV. It's not possible (or
at least not convenient) to separate the power and signal wiring to
provide a proper single point ground.

Both the sense and current limiting resistors are simply lengths of
#14 copper wire with forced air cooling. This works very well with
the diode's output digging pits in my brick beam stop. :) However,
for continuous operation, it may be necessary to replace the #14 with #8
because even the modest heating of the copper changes its resistance
enough to noticeably affect current.

With minor changes in part values for the current limiting resistors,
and the set-point for the power supply output voltage, it should be possible
to drive a pair of laser diodes in series as long as they can be isolated
from the common point. (The positive connection to a high power laser diode
is usually the mounting block of the diode but it may not be connected to
the external case itself.) However, one risk with this setup is that if
one of the laser diodes fails shorted, it will likely take the other one
as well since the current will spike to a very high level.

The setup is shown in Photo of Sam's High Power Laser
Diode Driver In Action. The water-cooled laser diode in the aluminum box
is capable of 35 W output at around 55 to 60 A.
The power supply is at the upper left with
the control panel in front of it showing 40 A. Behind the power supply
is the coil of white wire acting as a current limiting resistor next to
its cooling fan. The current sense resistor is the 12 inches of so
of red wire running from the power supply to the terminal strip. The
blue-white glow is my digital camera's response to intense IR.
The camera is really confused. :) When viewed through IR blocking
laser goggles, a line on the brick starts glowing at a current of
around 35 A and is white-hot at 45 A, where the current limit of the power
supply is presently set (via the current limiting resistor and wiring
resistance with the power supply adjusted for a maximum output of 5 VDC).
The old darkroom enlarger timer in the upper right is used to turn the
driver on for exactly the 20 seconds needed for my "meat thermometer"
type power meter to take its reading, which would show about 23 W at 40 A
for the diode in the photo. The reading at 45 A is about 27 W.

Tim's High Power Laser Diode Driver (TO-LD1)

(The schematic and portions of the description below are from:
Tim O'Brien (ob1@xtra.co.nz).)

The circuit in Tim's High Power Laser Diode Driver
is designed for high power laser diodes which include a monitor photodiode
for optical feedback. Note that most common high power diodes are driven
with a constant current but optical feedback enables more precise control of
output power. Diodes like this are available from
Roithner Lasertechnik at very
reasonable prices.

The front-end is a current differential amplifier (very similar to the
approach used in the LM2900 Norton op amp). I hand-picked
the two transistors for the current mirror for close matching. They are
mounted in a common heat sink to keep them at the same temperature.

The constant current sources are LM334s. These are cheap and work well.
The one used on the non-inverting input of the current mirror is adjustable
to about 2 mA. The one used as the common emitter amplifier load was set to
about 1 mA.

There is a 100 uF, 16 V capacitor on board too as well as a reverse
biased diode in parallel with an RC snubber directly across the laser leads
(not shown).

Digitally Controlled Laser Diode Driver

"Visible-Laser Driver Has Digitally Controlled Power And Modulation" was
published in the "Ideas For Design" section of "Electronic Design", March 23,
1998, by Roger Kenyon of Maxim.
Go to http://electronicdesign.com/"Electronic Design and search
for "visible laser diode driver digital 1998" or something similar.

The circuit provides 1024 discrete output levels from a laser diode (with
optical feedback) using a D/A converter with a 3 wire serial input. In
essence, it is a basic laser diode driver with a programmable reference.

Pulsed Laser Diode Drivers

The following circuits would be suitable for driving the type of pulsed laser
diodes found in the Chieftain tank rangefinder and currently available from
OSRAM Opto Semiconductors and possibly
other sources. These are very different than the sort of laser diodes with
which we are generally familiar. A typical specification might be 8 W peak
power at 850 or 900 nm (depending on model) with power requirements of 10 A
at 0.1% maximum duty cycle. Thus, the average output power is actually
in the mW range even though these laser diodes may be listed in some surplus
suppliers' catalogs (like those of Bull Electronics) as multi-watt devices
with the duty cycle restriction listed in fine print, if at all! Since the
average power dissipation is also very low, they may come in plastic packages
like LEDs with flat polished faces (and no possibility of adding a heatsink,
which is one of the major limitations on average output power)! Other than
time-of-flight laser rangefinders and related applications, I'm not sure what
use these would be to a hobbyist. And, their output is totally invisible
but very definitely not eye-safe.

Here are a couple of options for drivers:

A simple approach that should work is to use an SCR as the switch
triggered by your favorite pulse generator, 555 timer based astable, or
other oscillator circuit followed by a trigger device like a neon bulb, diac,
or small SCR to guarantee fast turn-on of SCR1. The circuit below is similar
to the one from Scientific American (see below) which describes the use of
pulsed laser diodes back in March 1973 when no other types had been invented
yet (or at least none were readily available). With the component values
shown, the laser diode should have a peak current of about 10 A with a
100 ns time constant. Thus, it isn't a nice rectangular pulse but that's
for the advanced course. :) R1 limits charging current, R2 limits discharge
current, and D1 provides reverse polarity protection for the laser diode.

Scientific American had an article on driving a pulsed laser diode in
"Infrared Diode Laser", March, 1973, pg. 114. This is also a part of the
collection: "Light and its Uses".

There is a pulse drive circuit in Skip Campisi's "Laser Clinic" article
in Poptronics, June 2001. It's based on an NPN transistor operating in
avalanche mode to generate the required short high current pulses.

There used to be a driver circuit on the SVBx High Tech Labs Web
site without attribution. (However, this Web site is now defunct. If
anyone has saved this circuit, please send me mail via the
Sci.Electronics.Repair FAQ
Email Links Page.)

The RCA SG2002 laser diode is probably long obsolete but the ones found in
Chieftain tank rangefinder should be similar (though the specific
ratings may differ somewhat). OSRAM
Opto Semiconductors currently manufactures similar devices.

I couldn't find a substitute for the VM64GA but I expect that a readily
available N-channel enhancement mode MOSFET like the IRF530 would work in
its place. Replacements for the any of the other parts shouldn't be critical.
Make sure you have the complete datasheet for your laser diode so you
can modify component values intelligently! :)

The discrete totem pole buffer circuit designed to provide very fast turn-on
and turn-off may be overkill depending on your requirements and it may be
sufficient to just drive the power MOSFET directly from a pulse generator
or other signal source.

Check out Directed Energy,
Inc. for schematics, white papers, and specs using ultra fast power
MOSFETS. You can also buy complete drivers for pulsed laser diodes
with pulse widths down to at least 4 ns at 40 AMPs.

And just to repeat, in case you have forgotten: Most common low power laser
diodes can't be pulsed in this manner to achieve high power status - they
instantly turn into Dark Emitting Laser Diodes (DELDs) or expensive LEDs. :)

Hewlett Packard LaserJet IIP Laser Diode Driver

(From: Rob Kirke (r.kirke@sepsretravision.com.au).)

I just recently reverse engineered the IR laser driver
out of an HP LaserJet IIP (Part number RG1-1594). I've drawn up
the full schematic for the board and have got it working outside the
printer with with a simple power supply using a 7808, 7805, and a couple of
capacitors. See Hewlett Packard LaserJet IIP Laser
Diode Driver (RG1-1594).

The board obviously supports very fast beam modulation, and has a complete
collimating assembly. The diode itself is a standard case, and easy to get
to (4 screws, no glue or springs) so it could be swapped for a visible
diode or replaced easily. Two feedback adjust trimmers are located on the
board (one fine adjust, one coarse)

Also, I've seen these boards advertised as replacement parts on the net for
$20, so they would make quite a nice unit for someone who doesn't have the
time to build a driver board up.

Here is the pinout:

+5 VDC

GND

Photodiode output

Laser drive level input

Modulation (Active low)

GND

+8 VDC

The feedback loop seems to be 1:1 so pins 3 and 4 can be shorted together
(Mine runs at about 43 mA under these conditions). Pin 5 was originally
driven by a single gate from a 74LS08.

Hewlett Packard LaserJet IIIP Laser Diode Driver

(From: Filip Ozimek.)

The pinout of the LaserJet IIIP driver is the same as for the LaserJet IIP,
above. I found that light emitted from the laser diode is
786.5 nm (measured with spectrometer) and average power is about 4.5 to 5 mW
(measured with a laser power meter). The laser diode is enclosed in a TO-18
(5.6 mm) package with ground connected to the case.

Description: This laser diode modulator exceeds the performance of
many, for a few reasons. First, the LM317 regulator, though not
approved by most experts for this task, is clearly aimed at excellent
transient handling, and is known for this ability. If 1.5 amps is not
enough, you can substitute for the LM317 with an LM338 to get up to 5
amps, and even gang several of them (as described in the data sheet,
and see the detail, below), making huge output power available with a
very simple circuit. Graphs in various data sheets support my
assertion that it should be fine up to at least 1 MHz with arbitrary
waveshapes, and there is a radio engineer called Harry Lythall who
accidentally made an LM317 oscillate and subsequently transmitted
successful messages at 1.8 MHz and who thinks it can be pushed to
higher frequencies. (I think that with a sine wave requirement for
radio broadcast, he is certainly right about this, but I have no ham
license so I won't be trying to prove it).

How to use it: Set the pot to maximum, remove modulation from
input. Switch on power. Adjust the pot downwards till the beam all but
disappears as the diode current drops just below lasing
threshold. Then apply modulation. That's it.

Details of use: Build the board as described in the second picture
(some details from the main text below will be needed to do this
right). LM317-Based Laser Diode Driver
Artwork can be used to make the PCB. If you can't get an LT1215
op-amp use a CA3240 for now. It's slower but it works well if about
200 kHz is all you need (it will still do this better than the
Die4drive or Flexmod N2 drivers). To get fast modulation, to well
beyond 1 MHz, a fast slew rate is needed. Large gain bandwidth product
is less important because both halves of the op-amp are running at
unity gain, but stability at unity gain is important. To optimise the
compromise between stability and speed, some small ultrafast ringing
occurs, but nothing that harms a laser or disturbs a show. The LT1215
needs several picofarads of compensation for unity gain stability (one
capacitor for each stage, a tiny surface mount device soldered to pads
on the back of the board). For speeds below about 200 kHz we can get
away with a CA3240 (which needs no such compensation), making this by
FAR the easiest laser driver to find parts for! But we want a FAST
laser driver, do we not? :)

Wire the pot so the ground is on the top end of the scale, a fully
clockwise turn should ground the wiper. Use a GOOD pot, a sealed
cermet or polymer track type. Even better, a potentiometer IC which
offers several useful advantages for remote control as well as low
noise and long life and secure retention of setting... At minimum it's
all modulation, at maximum it's all fixed output current. The board
has space for a single turn cermet preset pot, and four pads, so it
can be mounted such that full clockwise turn can be all modulation, or
full fixed output current, depending on preference.

Any input subtracts from the reference voltage, and full input almost
cancels it (but not quite, it it wise to avoid ground clipping in the
op-amp if possible, to prevent strange behaviour at high speeds). When
this inverted modulation is passed to the second stage via the pot, it
subtracts from the load's high side voltage, making the regulator
reduce its current when the original input goes low. At full input or
pot maximum, as either case removes input to the second stage
inverting input, the second stage output equals the load's high side
voltage, which in turn means the maximum current is solely determined
by the regulator's fixed resistor. The input's zener diode prevents a
signal capable of driving the regulator to output more current, as
does clipping at ground in the first stage even if the zener failed.

The pot is also useful as a simple fader in absence of modulation, and
if set to minimum position, allows remote dimming by varying an input
DC voltage from 0V, to 5V for full output. Note that local dimming
with the inbuilt pot overrides the modulation at maximum setting, you
cannot modulate to reduce from maximum preset brightness. If you need
to do this, as with some inbuilt laser modulators, you need to invert
your signal before input to the driver, as is usual in any laser
system that conforms to high side drive and active high input. Usually
you'd only need to do this when the laser's inbuilt modulator does not
conform.

The second op-amp stage takes into account whatever the laser diode
voltage drop is, so any dynamic shift in that value with changes in
current is automatically compensated. Dangerous input, including
overvoltage, reversed polarity, even moderate static discharge, is
prevented from harming the driver or laser diode by a combination of
input resistor and zener diode. The zener's capacitance also forms a
filter with the 100R resistor, that allows clean waveshapes at 1 MHz
while slowing down any transients fast enough to cause trouble later
if they got through. Don't tempt fate by being careless of static
discharge, otherwise add a large 6V metal oxide varistor across the
input socket if you insist on riding the lightning. C3 and C4 on the
board are for power supply decoupling. Make C3, the closest one to the
op-amp, a 0.1 uF low ESR ceramic, and C4 a 40 V, 22 u low ESR tantalum.

Another important aspect of the use of two op-amp stages is the repair
of mark/space ratio symmetry at high speed due to differences in rise
and fall time. As one stage falls when the other rises, the
differences cancel to an extent great enough to allow higher
performance than with a single stage. Despite two stages slowing the
transitions more than one, this is still true, as scoping the output
of the first stage, then the second, will demonstrate. A small
asymmetry exists at 1 MHz anyway, but far less than appears on the
first stage.

NOTE: An SPDT switch could select either the input on the first stage,
or the output from it, to feed to the pot. This could allow a simple
way to get compatibility for systems with inverted signaling, but the
price will be a loss of symmetry correction for high speeds because in
this case the first op-amp stage is not used.

As you can gang output resistors and diodes to one regulator, and
multiple regulators to ONE driver, you can set the resistor for any
diode that falls outside spec for a given batch. You can set control
for THOUSANDS of diodes safely with ONE potentiometer, if you want to,
so long as they have the same ratio of maximum current to threshold
current. For any diode's resistor, calculate value by 1.25/A where A
is required maximum current. Pick the nearest preferred value ABOVE
the calculated value, then calculate the required resistance to
parallel with it, to get the total resistance needed if the initial
value is more than 1% out. Calculate to find the power dissipation for
each, so they won't burn out in use. The board layout has space for
this, when driving a single diode up to at least 1.5 A, but bigger
resistors may be needed if using an LM338. The circuit MUST monitor
the high side of ONE laser diode, but other diodes and resistors can
safely use whatever current the regulator makes available to
them. Ganging regulators is the same, common the ADJ pins, you still
only need to monitor ONE laser diode. Ideally, pick ALL duplicate
components from the same high grade batch, and leave a little headroom
to allow for slight differences. The odd loss of a laser diode, if you
like to push the envelope, is MORE than made up for by the ease of
building a simple array extensible by few, and cheap, parts. This is
one of the joys of a proper high side driver.

No matter how big the laser system, you only need three drivers, one
per primary colour (plus one for any unusual extra colours you might
use, such as 405 nm). You do NOT need one per diode here! :) Given the
quality of the ideal potentiometer, this is a Good Thing, those
cost. But, less than a diode driver costs so this still saves
plenty. Remember that the high load voltage capability (up to about 3V
short of supply, which in turn can be up to 36V) means you can string
laser diodes in SERIES from one resistor too, as an alternative to
parallel connection. With this driver (fitted with LM338 instead of
LM317) you could likely modulate a 32V 100W floodlamp LED with a 1 MHz
sawtooth wave, but I can't think why the hell anyone would want to.

(From: Sam.)

I have tested a handcrafted sample of one of these using a dummy load
(2 silicon diodes in series) and that worked as advertised with no
overshoot when driven with a squarewave and a frequency response to
at least what is described above. While this is not exhaustive, the
results do seem promising.

Laser diodes need stable drive, especially when driven close to their
safe limit. High power DVD diodes and multimode diodes may mode hop,
so controlling power is less feasible than controlling current. This
is because there may be at least two specific currents in some range
that can produce a given output power, and is also why these diodes
rarely come with photodiodes built in.

This means that the only other significant thing we can easily control
to maintain drive close to safe maximum output is the diode's
operating temperature; or in this case, the current with respect to
whatever that temperature happens to be. Then, barring mode-hops, the
output of a direct injection laser diode is stable enough to predict,
which means that all we really need to do is specify the maximum
current for the diode's own safety, at the minimum temperature at
which we will run it.

Then again, there are those who like to use LM317-type circuits
because they're cheap, powerful, accurate, and can even be modulated
to 1 MHz or more with any waveformm. We may also like to push for the
best safe current at ANY temperature (within reason). As all these
circuits have one thing in common, a sense resistor, the best place to
easily compensate for temperature is to modify this resistance
directly. Here is a way to do it.

This is a bit terse, but complete in detail. (Reading on the net about
thermistor selection will help too). What it does is use an array of
two or three surface-mount negative temperature coefficient (NTC)
thermistors, bonded to the laser diode's heatsink, and connected
directly to parallel the sense resistor. Apart from taking this
thermistor resistance into account when calulating current for any
LM317-based driver, no other modification to any circuit is needed.

An example: The Opnext/Hitachi Laser Diode HL6526FM

The current for 80 mW output at specified temperature was measured as:

Here are the equations for deriving values from the technical data, 1K scale,
2.326K at 0°C and 0.49K at 50°C:

T = K(R/1000) T is value of the compound thermistor at given temperature,
K is the known value of a 1K thermistor at given temperature,
R is the known value of the compound thermistor at 25°C.
1/F + 1/T I is the resulting current in amps supplied by the regulator.
I = --------- F is the value of the known fixed resistance,
0.8 T is carried over from before.
P = T(1.25/T)^2 P is the resulting power dissipation at given temperature.

If T is 75 ohms, made from two 150 ohm NTC thermistors in parallel, and F
is a network of metal film resistors equal to 12.4 ohms at 1% tolerance,
the regulator tracks the current/temperature curve of the laser diode
extremely well, and the total regulator parts cost is less than £2.00.

0°C=8.96 mW They will be dissipating a lot more power than the optimum 1 mW,
25°C=20.8 mW but well within specs for the operating temperature range. They
50°C=21.3 mW will be thermally clamped, bonded to the laser diode heatsink.

Note: While scaling is excellent, the offset isn't, given only 5% tolerance
in the thermistors, so a 1% metal film resistor network must be built with a
variable resistance to finely tune it. The statement
(1/75+1/(1/(1/(4*3.3)+1/(100+470))))/0.8 allows a 470 ohm multiturn preset
to set the current between about 4 mA and 6 mA off-range, just enough to
accommodate a 5% error. While it looks safe, the calculation predicts a
very non-linear response, so this one must be tested. The main resistance
will be four 3.3 ohm resistors in series, parallel with a network of series
values, 100 ohms and the 470 ohm preset. Which is wired so the current
falls, not rises, if it goes open circuit.

Parallel Laser Diode Driver 1

(Circuit and portions of description from Szabo Gabor.)

Unlike most other laser diode driver circuits, this one essentially robs
current from the diode so that it should be able to be added to most
existing constant current systems without modifying their design. Use with
caution though as they will see a dynamically varying load, so AC or
transient behavior may not be predictable. See
Parallel Laser Diode Driver 1
This circuit can operate with modulation because of the leaky integrator's
averaging effect. The time constant can be changed by modifying the 3.3 uF
capacitor. It's currently set to ~15 ms for our 78.125 kHz modulation,
but in CW mode there's no need for this capacitor at all. This example uses
light feedback, but that is not necessary. Another benefit of schemes like
this is that it should be impossible to damage the laser diode from overcurrent
unless the constant current driver becomes unstable. Of course, it must be
set at the highest current desired. Then the parallel circuit can reduces
current from there.

More Laser Diode Driver Schematics

Here are a few more. Some have errors!!! See notes below. And the circuits
found in the manufacturer's application notes are often not tested. :)

Skip Campisi has a nice article entitled "Laser Clinic" Poptronics, June
2001. There are schematics with complete parts lists with component values
selected for the Sharp LT022MC 780 nm LD, Mitsubishi ML720 1,300 nm LD, and
the Hitachi HL6712G 670 nm LD (all 5 mW max) and a pulsed driver for the high
power LASD59 (similar to the RCA 40861 and LDs from OSRAM and others).

CAUTION: While the author does provide some basic laser safety information,
it would have been nice to have more on the the critical drive requirements
of laser diodes. I'm afraid there may be some disappointment when more than
a few laser diodes turn into DELDs. He notes the effects of ESD and reverse
polarity but doesn't appear to deal with the very important maximum current
ratings. The only way to set up these laser diodes for maximum safe (for the
LD, that is) output is with a laser power meter since their characteristics
vary from device to device.

Circuit Cellar Magazine has a
design using a PLD that will drive a typical low power laser diode using
optical feedback and includes modulation. See:
Project 247:
Laser Diode Controller. It can probably do a lot more than they have
implemented without requiring additional parts. However, circuit to simply
provide the features shown would only cost about $2 for discrete parts
or a laser diode driver chip, no downloading of firmware needed! I'm
also not convinced it handles power cycling or fault conditions reliably.

Communications Systems Using Diode Lasers

Raw laser diodes typically have an electrical->optical frequency response that
extends to hundreds of MHz or beyond. However, most simple drivers designed
for continuous wave (CW) operation (including all of the discrete circuits
described elsewhere in this chapter) have such heavy filtering and isolation
from power supply transients and noise that control beyond a few Hz is usually
not possible.

In principle, modifications to improve the frequency response by reducing the
filtering, and to provide a modulation input, should be straightforward.
However, in practice there are all sorts of ways to screw up resulting in
either unacceptable behavior or a dead laser diode or both and it is usually
much better and easier to drive the laser diode in such a way that it never
goes into complete cutoff:

(From: Jonathan Bromley (jonathan@oxfordbromley.u-net.com).)

"I'll second that. Modulating a laser to complete cutoff is a very, very bad
game for all sorts of reasons:

The light versus current behavior is hideously non-linear below about 10%
of full output, so you really need dynamic feedback control - but the
photodiodes tend to be slow, so that's not on.

Spectral quality, and beam shape, go to blazes at low power levels

lasers don't turn on from fully-off anywhere near as fast as they can vary
intensity around the 50% level.

It's pig-difficult to design the modulation circuit so it is guaranteed
never to overshoot the current that gives 100% full light output (which is
essential, because even very brief over-power transients dramatically shorten
the laser's life).

But if the information from Honeywell and others is to be believed, the new
vertical-cavity surface-emitting lasers (VCSELs) are much better behaved and
can be modulated to extinction at quite high rates. They're also extremely
cute bits of device technology.

A couple of simple such modulation circuits are shown below.

CAUTION: Since both the following affect the optical feedback, attempt at your
own risk!

The following applies to laser diode power supplies TO-LD1 and RE-LD1 through
RE-LD3. Similar modifications could be made to RE-LD4 but this is left as an
exercise for the student! :-)

A bi-level modulation scheme could be easily implemented by connecting a
general purpose NPN transistor across an additional resistor (at point XY).
Then, full power will be achieved with the transistor turned on and reduced
power with it turned off. Select a value for R2 that will still maintain
the current above the lasing threshold - 1K is just a start.

Here is another circuit which should achieve somewhat linear control of laser
power since optical power output should be proportional to photodiode current.
Resistor values shown are just a start - you will need to determine these for
your specific laser diode and operating point.

Peter Philips'
Laser Link Communicator was originally published in "Electronics
Australia", July 1997. This allows for the transmission of high quality
audio up to distances of several hundred meters. Either a visible or IR
laser diode may be used (the latter providing for greater security but
increases the difficulty of initial alignment).